My dad is planning to run away from home. What can I do to stop him?

If he has access to a car, disable it completely ASAP. Make sure it is not something that can be fixed easily or cheaply. I agree he needs to be in memory care that is locked. I know that sounds harsh, but it is what I had to do for my Mother’s security. I was well advised she would not be safe in any facility that was not secured. I checked out a few before making the selection and it was quite nice and the best thing for her. She was free to walk around the facility wing. Walk out into fenced in courtyard, tv room and be apart of activities.

My brother in law and sister in law where moved to AL when their son felt their mental state was normal. It wasn’t. Wife’s Alzheimers state was worse then his, but he was in beginning stage of Alzheimers. On the 2nd morning, they did not show up for breakfast, so the staff checked up on them. Their car was gone. No cell phone. It was 2 days before they were found. Thankfully safe but confused.

When we lived in FL it was more then once a golf cart driver was found on Hwy 75 driving their golf cart in the lane of traffic, not knowing where they were.

Based on your comment, "His last trip was an excursion on a 3 wheel single speed granny bicycle. He went 30 miles across a mountain on an unimproved road. He was found dehydrated, disoriented and spent several days in ICU. He claims no memory of the event.
This "trip", he asked a family friend for a ride. To my knowledge he doesn't know anyone at the destination. He is planning in secret and then just disappears without warning. We never know if he is in a ditch or on an adventure.
If he wants to travel, I don't begrudge him that. It's not fair that he leaves and worries everyone unnecessarily. Involving friends, neighbors and police in a silver alert/man hunt is terrifying."

You need to realize your father has more than 'mild dementia' going on, and nearly died on his recent 'trip' which you don't want to 'begrudge' him of. He does not have the mental ability to take these 'trips', to leave on his own, and if you continue to allow him to do so, he is going to die.

Dementia is a brain disorder; the brain no longer functions properly, as evidenced by his bicycle trip. Understand and believe this statement to be a fact. And, as such, treat it with the seriousness it deserves by placing your father in a Memory Care Assisted Living environment where he is unable to leave the premises.

Otherwise, his next trip may be to the morgue. I used to care for an elderly man with dementia who believed he lived across the street. His daughter 'loved him too much' to place him in Memory Care, and felt he was entitled to live in his own home & do as he saw fit. Which he was...........and he was also entitled to pay the consequences of his actions, I guess. Because one night, he left the house, fell in the street & wasn't found for a few hours. When he was taken to the ER by ambulance, he was too far gone to be saved. He had a subdural hematoma and died the next day.

Your father is not capable of taking a 700 mile trip. Do something to prevent it before it happens. Either that, or accompany him on this trip so you can care for him during it and make sure he's not doing something to endanger his life. He can't use reason anymore; dementia prevents it.

Wishing you the best of luck managing your father's care.

Does he own a smart phone? If so you can install a tracking app so you at least know where he is (police can get some idea of his location by the last cell tower it pinged even if the phone is turned off... this also is true for an analog phone, like a flip phone)

Does he carry keys? You can put a Tile on it that will also track him, just don't tell him what it is.

The title of your post tugs at my heart, I can't imagine the worry you are feeling. Was he an adventurer in his younger days?

He seems like he is passed Independent Living. He needs Memory Care.
The fact is in Independent Living they can not stop him from leaving. He could very well "wander out" at night to go for a walk, go to the store, or anywhere he pleases.
If you, the family does not wish to move him to Memory Care then part of this is on you. Yo can place a "tracker" on phone, in his wallet or other piece of clothing. At least that way you can tell where he is. Some you can set to a particular range so if he leaves the area you will be notified.
But the only way to prevent him from leaving is to transfer him to a locked MC unit.

Is your father so advanced in dementia that you cannot speak with him about this, that he cannot retain anything you say?
If he CAN understand I would explain to him that the next time he endangers himself in this manner he will have to go to a locked memory care facility.
If he cannot understand or retain, then in fact it may be that time. Such a trip could end in his death. Easily. My friend's brother disappeared, and was found across the bay very lost, and prey to all. Luckily found. Many elders are not found every year.
I am so sorry you all are going through this.
There are some police bracelets available in some areas and I would discuss with your local department. My brother's ex wore same when he was not in locked unit, used to disappear for alcohol.

Your father is passed Independent living. He needs an AL where he can be monitored more closely.

Your father has mild dementia now; but then you state that over the past ten years he has "run away from home a couple of times."

In what sense was he running away? Why wouldn't he have been willing and able to discuss these adventures with his family beforehand? I suppose I'm wondering why you can't just ask your father about his travel plans in the ordinary way.

His last trip was an excursion on a 3 wheel single speed granny bicycle. He went 30 miles across a mountain on an unimproved road. He was found dehydrated, disoriented and spent several days in ICU. He claims no memory of the event.
This "trip", he asked a family friend for a ride. To my knowledge he doesn't know anyone at the destination. He is planning in secret and then just disappears without warning. We never know if he is in a ditch or on an adventure.
If he wants to travel, I don't begrudge him that. It's not fair that he leaves and worries everyone unnecessarily. Involving friends, neighbors and police in a silver alert/man hunt is terrifying.

Your profile says he is in independent living. If so, then what do you mean by "running away"? Do you mean he's planning a vacation? It's not the same thing but I understand your concern if he has memory issues or his decision-making is less rational. If he does not have a dementia diagnosis and you are not his legal PoA then you can't really stop him. How is he planning on traveling: by car or plane or someway else? Is he aware of the risks of covid? You'd need to talk to whomever will be hosting him at his destination to make sure he's safe.

If he forgets his medication would it be life threatening for him? Can you offer to go with him if he's really determined? How is he making the travel arrangements? Is someone there helping him? If you could provide more info it would help the forum participants to provide better suggestions.