Is there an app or service for people living alone that alerts someone if the loner doesn’t check in?

I live in a small home town. I asked the very same thing. I received an unusual answer that was interesting and easy. If a neighbor noticed the porch light not turned off by morning to call or check on them.
I now do that with my neighbors.
also, I keep my car clicker with my alarm close to me. My neighbors know if the car alarm sounds non stop. Call 911 for me because I cannot call.

I was a city mail carrier. There were several individuals along my route that we knocked, opened the door, called in “Are you okay today?” Placed the mail where we were previously instructed, and reported back to the postmaster. In our area, country mail carriers also meet special needs for individuals who cannot walk to their mailbox. I know that special arrangements can be made. I would also connect with the Commission on Aging if one exists in your community as well as a local church if you are affiliated with one. Lastly, there are several electronic devices, one you wear around your neck, that alerts someone to check on you immediately. I hope this is helpful.
CSS

If you are eligible for meals on wheels, then I would sign up. MOW will check up on people once a day and provide a hot meal.

Yes, there are apps and appliances you can use. But you would be better off creating human connections. This is not just an issue for seniors.

Some examples:

Mil has grocery delivery, if she does not call in her order on Monday, they call her.

Elderly Aunt, had her Postie knock on her door and hand her her mail each day, he was the one who found her when she broke her hip.

Have your paper boy knock on your door.

Call a friend every day, by a certain time. It can be a different friend each day. But have a plan in place if you are unable to connect.

As a culture we are losing our human connections. There was a time that if the milk bottles were not brought in, someone would knock on the door. Of course most of us no longer have milk delivery, but we have also lost daily human contact.

Perhaps it is a service that needs to be developed and provided. Outside of home care, but a friendly call each day.

Found a couple, I don't know how much they cost but maybe these are what you are looking for?

https://play.google.com/store/apps/details?id=com.chkincam.fall&hl=en_US (Check in and fall alert)

https://snugsafe.com/ (Calls you once a day to check up on you)

When we had to keep an eye on my in-laws I met (secretly because in-laws were in denial) with their next door neighbor and made an arrangement: the neighbor would bring the newspaper to them personally every day since we didn't want our LOs to venture to their distant mailbox in the icy MN winters. I gave the neighbors permission to read the paper first, as a thanks. I also gave them grocery store and Target gift cards to use on my in-laws behalf to keep them stocked and prevent them from driving. I agree that we need to fold in friends and neighbors as much as possible. In particular, if you belong to a congregation or club, try to pick a "point person" to check on you. If there's someone you trust you can give them the secret location to your house key (which I hope you have somewhere outside) so they have easy access for a wellness check. If you live in a small town you can let the police + fire dept know you are a single senior.

There are wearable alert devices that ping the control center if it detects the wearer is horizontal at an odd time of day or for an inordinate amount of time. I don't know the brand but I know they are a thing. My very seniors aunties in FL both have the alert necklace and they've used it when one of them fell. Worked great!

You must have a computer or a phone to be participating in this e-conversation, so use it to say “good morning”
to several friends (the same ones) each day when you rise. It will be a pleasant way to begin the day, and you’ll all be safer.

If you have friends and family with cell phones you can create a group message and text the group every day. Include the date so they know it's current and not a message that's been delayed. "Hi, it's Frances checking in on Nov 19, 2019. Have a great day!" And request a response so you know they got it.

Simple. Uncomplicated. Something everyone in your circle can do without installing or buying anything.

Although there are lots of motion sensors out there, I'm not aware of a system set up to alert on "no motion". When I worked and left the home before my mother got up, I had a motion detector on her bedroom door setup to send an text message when the door opened. If I didn't get one by 9:00a, I called her. I'm sure you can get some detector to meet your needs, but you may need to engage a monitoring service or person to get the protection you desire.

I have heard of some senior centres that have people [volunteers] who make daily calls & if no answer then there is a process

This may show that you are a bit isolated - as a new retiree start checking out what is available to get you out more where your absence would be noted

I heard of 2 sisters who would phone each other at 9:00 AM every day even if it was 1 minute just to know they were fine but most calls were longer - can you do this with a friend or family member & they don't need to live in same town with much cheaper/included long distance rates now