My mother has Alzheimer's, more on the severe case. She walked out of the house the other night and neither my father or I knew. Any suggestions?

Aznbobbo, that is scary when a love one wanders away from home, thank goodness a neighbour found her.

Here is simple suggestion. Put black rugs in front of the exterior doors, on the inside floor. To Mom, those black rugs may look like a dark hole in the floor, thus she may be afraid to step on it. This may or may not work. It's the less expensive thing to try.

You can do a security system that chimes when doors are opened.

If you or someone you know is clever you can do this yourself for pennies on the dollar of having it done. Everything is remote now a days and that simplifies it tremendously.

I have 2 keypads, 1 near the bedrooms so it can be heard at night.

Here are articles about wandering: 
https://www.agingcare.com/articles/products-and-strategies-for-managing-dementia-wandering-142801.htm
(Please note that I do not think the suggestion of locks on the outside doors is appropriate.)

https://www.agingcare.com/articles/wandering-alzheimers-patients-142875.htm

Visual deterrents such as STOP SIGNS that hang ACROSS DOORWAYS and Black or DARK MATS on the floor that are interpreted as "BLACK HOLES" are a couple of ways to minimize wandering of people with Alzheimer's and Dementia.

Here are some websites (Copy & Paste URL to your browser.) of Stop Signs made especially as visual deterrent for wandering. Many of the signs are attached with Velcro-like fasteners so that you can take them off anytime you want to.  The sign will need to be removed prior to your loved one going out the door. 

https://www.webmd.com/brain/10-ways-to-prevent-wandering#

https://www.alzstore.com/stop-sign-banner-p/0134.htm

https://www.caregiverproducts.com/posey-stop-sign-door-banner.html

https://www.mindcarestore.com/stop-sign-banner-p/mc-0134.htm

https://www.alzstore.com/alzheimers-dementia-wandering-s/1828.htm

“Locator Services for Wandering Alzheimer's and Dementia Patients” Several companies offer (for a small monthly or annual fee) “medical alert systems” for dementia patients with or without GPS. 
This website offers a list of 10 Lifesaving Location Devices for Dementia Patients:
https://www.alzheimers.net/8-8-14-location-devices-dementia/

You could buy some "open door indicators?" similar to those used by business, that sound whenever the door is opened.

Good Luck with finding an option that works for your family. {{{HUGS}}}

Medical supply stores have products that you can place in the bed, a light layer, that nobody can feel as uncomfortable, and even on chairs, if there is a recliner or lift chair for your LO, that will ring an alarm when someone tries to move out of the bed or the chair. Problem is, they are very sensitive things, so they may ring the alarm more than you want.

I don't know if anyone has this product, but someone should. If you can place an area rug on both sides of the bed, a thin one, one that nobody would trip over (tape it to the tile or carpet, whatever), and place a alarm based thin liner under it.......that would always let you know when your LO got out of bed once you were off on your own in bed.

I purchased an inexpensive alarm system for our house. It works. It has wireless switches at the doors and a central unit that just plugs into a wall outlet and I also ordered a pager that goes with it. It is a local pager that I wore around the house. No service company involved. It worked for us. but you have to live in the house for you to be contacted. It has a list of other attachments that can be added to it, like mats and motion sensors.
I also installed some little flip locks on the doors and Luz never figured out how to use them.

There are basically reverse alarm type systems that have an "eye" that will alert you when someone walks by it so when someone goes through the door for instance you can get an alert on your phone and other devices. There is also at least one system that incorporates a bunch of things including an easy to use computer screen type thing that enables all kinds of access but I think that's more for elders still living on their own who need some supervision just not necessarily hands on but it has a monthly charge and I don't think it's inexpensive. Some of the other devices that you can monitor through your phone don't need a monthly fee. We have a camera that looks at moms parking area that detects motion and sends an alert so we can look to see who has driven in or out it would work on the front door too, I got it pretty inexpensively from Amazon. We also recently installed a Ring doorbell, they have several types of devices I think, and it sends an alert when it detects motion in front of it and anytime the door is opened and a different alert when someone rings the doorbell. That would work well for your purposes too, it's more expensive but also allows us to communicate with someone at the door. You can purchase cloud storage for both of those I think for a monthly fee but you don't need that to have it work, especially for your purposes I would think. Our camera over mom's "medicine" table stores 24 hrs worth of clips taken whenever it senses motion and we can watch it live anytime, I don't have alerts on that.

Moms door had a round knob. So I bought baby covers. They just go around and around. Theres a way to open the door but Mom never found it. My cousin put dead bolt key locks on his doors. His Dad was never alone. After he passed, cousin had locks removed. Had to do with the fire code.

Ring Door Bell.

Zmodo motion detection camera costs $30 @ Walmart.com. No monthly charge. Download app on your phone or as many phones as you like, and when motion is detected your phone will alert you. This had been great for my dad with dementia.

If feasible, install a lock that requires a key to open from inside the house and then keep the key with you. I do that at our house because my husband sometimes wants to leave at night.

There are door alarms that can be purchased. I picked mine up at a Home Depot.
However, what I found most useful were sleigh bells.
Dad had an antique ring of sleigh bells (from somewhere in his past) that I looped over the door.
He didn’t question it and my mother could hear those bells if Dad tried to open the door.

Bless you and good luck. The road you are on is not easy. Been there twice!

I've read (didn’t need to use) if you put a lock higher than normal, they don’t think to look up. We were going to do this on all the outer doors but my stepfather never wandered

Someone NOW Needs to Take the Bull by the Horns, She will get Worse...Discuss Care at Home or a facility Best from the Rest.

"10 Lifesaving Location Devices for Dementia Patients" ... see ...
https://www.alzheimers.net/8-8-14-location-devices-dementia

All I did was fix several 'dummy' handles to the inside of the door, and my husband never managed to find the real one! you could put some on the hinge side as well ......

I put a curtain up in front of my door and my mother has no idea there is even a door there.

My mom does this too, here’s what we’ve used:
1. Doorknob lock with tape over it
2. Security system
3. Mini chimes on door
4. Flip lock up high
5. Chain lock

good luck!

When this started to happen with my dad, we knew it was time for secured memory care. We discussed options with the family doctor but those mostly came down to locking down the house so nobody could easily escape, which seemed pretty dangerous :( It seemed also likely that dad would get super-agitated and angry if he came across doors he couldn't open. He was at a high level of paranoia and thought strangers were coming in and out of the house.

My Dad had Alzheimer’s and would try to go outside unattended. We purchased door alarms at Walmart and placed them at the doors going outside and to the basement. You could also place one to her bedroom door and the alarm would wake you when she opened the door

Cardinal Gates Door Guardian or a bed alarm booth from Amazon. I had the door guardian placed at the top of door so mom can't reach it. It's perfect for keeping her inside.

You can also get a bracelet from the police department that is mointed.

Get a security door that needs a key to unlock. If she is anything like my Mom if the door is open she is gone! She could really move if given the chance.

I bought a mat that I placed outside her bedroom door that had an alarm. When she stepped on it the alarm went off. When the alarm went off she went back inside her room. A few minutes later she would forget about the alarm and step on it again. She never went passed the mat, just went back into her room. The alarm kept sounding until I turned it off. I later programmed it to play "She'll be comin' round the mountain when she comes" for the humor of it.

You can buy inexpensive motion detecting security cameras that sound when motion is detected from a phone or tablet.

My Mom took the bracelet off as soon as I put it on, she also tore off a necklace that had info on it within 2 minutes. As careful as I was with her, she was a runner. After the bracelet and necklace didn't work, I put washable tattoos with her info on it on the back of her arm. She never got out but I wanted to be sure, if she did, she had info on her person at all times.

I had the same thing happen with my mom, except it was the State Police appearing at my door. I went out and bought door alarms for the outside doors and they would go off when she opened the door, so I could catch her before she got too far, but it's really no way to live. It was going off 2-3 times a night and she would wander into my bedroom and my kids'. And you always have to ask yourself "what if by some chance I don't hear that alarm?" There are woods, ponds, swamps and streams near my home, not to mention people drive very rapidly down our country roads. We finally had to put Mom into memory care.

A security door with a special lock and key likely the only answer here.

We put a house alarm with adt I believe and it goes off whenever someone tries to leave. It helped us. Good luck

You can buy a super cheap tracker If your mom can wear that interacts with your phone that will help you locate her if something happens. In the hospitals, If they put alarms on their legs if they get out of bed. Can you lock her in her Room At bedtime?

we have a double door lock. the lower one that can be unlocked by your fingers and the keyed deadbolt which can only be opened by a key. my mom is adept at opening doors so we key it locked and remove the key and put the key in another location. all of our doors are keyed to the same lock. i have a safety lock on the sliding door to the back yard as well as a broom stick in the track that she doesn't seem to notice. Yes i agree the biggest scare is when you wake up and they are not in their beds or the house. This has happened to me and i found her in the front passenger seat of my car wanting to take a trip.

Our and Mother's house both have double locks on the doors. Plus at Mother's, we installed a simple hook and eye latch up high, that she can't reach, on the screened doors. She slips out when I'm distracted preparing supper. A couple times I found her walking up the street, and another time on the ground in the back yard trying to holler for help. She told me she doesn't like it, and I told her I didn't like finding her on the ground. 😕

I am glad that your neighbor found your mom and knew where to bring her to.
My mom does the same thing. I have found her in our garage at 2am because she needed to catch the bus to get on the boat. Something from her younger days. So to fix the problem, I got a motion detector that I put in our hallway, with the alarm in our bedroom, so if she goes down the hallway it goes off and it wakens me. I also have a baby monitor in her room, so I can watch her. Our sliding glass door has a lock on the top that she cannot reach so she cannot get out at night. These all good ways we keep her in. We also pad lock our gate so she cannot get out too. All these things to keep her in and she gets angry and tells me I am keeping her prisoner and keeping her from her family. That's ok. Main thing is she is safe and not out wandering the neighborhood getting lost or risking a fall on the sidewalk. Motion detectors can be bought on Amazon and so can the baby monitor. Doesn't have to be fancy, just something basic. I love mine. All things in place, doesn't work if systems not turned on. She hasn't noticed the motion detectors or the baby monitor in the room, so we are good there. Take care and God bless.

My niece and nephew, both physical therapists who work with Alz patients, told me to put either a mirror or a big picture of a book case on the inside of the door bc Alz patients will not walk through the door if they do not recognize it as a door (picture of bookcase) or if they see someone (their reflection-will not recognize themselves) standing in front of them. I am worried about my mother exiting the doors to outside, as well, and wander, like your mother did. Hope this helps and I pray it works. Please keep us all posted if you try this. Best wishes.