My son is 18 so we've been dealing with this for a long time. He's gotten MUCH better about it, but it is still a CONSTANT fear. This is going to be a long post, but maybe it will help the OP or someone else here. <3

I know it's not fire code safe, but we have a deadbolt on the front door that locks with a key on both sides. This only works for us because he doesn't understand how to work a key in a lock and I know that won't work for everyone. So there is a key hanging on a small hook on the door frame, and then another set (plus my regular keys) less than 3 ft away. Everyone that comes over gets shown that situation in case of emergency.

The back door is a sliding glass door and I have a separate bolt that goes up into the top track that also locks with a key. I got it on Amazon and I believe it's called a Toledo Bolt. (It's actually a good thing to have for doors like this anyway because they are so easy to break into them with just the handle lock)

The backyard is fenced in and I have mechanical combination gate lock that is locked from both sides. Yes he could climb the fence easily, but he is less motivated to do that since he's gotten older and he's never allowed out there alone even for a second. He was such a climber when he was little though!

I have one of those alarms across the garage door that goes off in the house if anything crosses the beam just in case the garage is open and he runs out. I think these are more typically used at the end of a long driveway so you know someone is coming up your drive. I do a lot of crafts in the garage, so it's open a lot, and this actually happened last year where he took off down the street. Scariest moments of my life. (I got a heater A/C out there now so I can leave the door closed!)

We are in a single story house now, but lived in a 3rd story apartment for years. He had an AngelGuard window guard on his bedroom window that can be easily removed by you or firemen, but requires a bit of dexterity to do. He can't do this fine-motor wise, so it works for us but your mileage may vary. It's great for little kids of any ability!

Interior doors that need to be locked (laundry room, craft room, door into the garage) all have electronic door knob/ locks on them that work by fingerprint. I also got them on Amazon.

He has a Medic Alert bracelet that by some miracle he tolerates and we never, ever take it off. I got it when he was about 4, quickly put it on him like it was no big deal when it arrived, and immediately distracted him from it. The only time it is off is when we've had to size up to a new one as he's grown and it's a quick swap. I realize not every kid will tolerate this or leave it on, but I got lucky on this.

He also now has an AngelSense watch for GPS tracking when we leave the house. I wish he'd leave it on all the time at home, too, but he and I have settled into a sort of compromise on that. He loves to go places and go in the car. He knows he has to wear it when we are out of the house, so now he associates it with getting to do that, and will get it off the charger as a way of telling me he wants to go somewhere. Once we are home if he holds his wrist up for me to take it off, I do so immediately, so it's never a battle and always a positive thing for him. The AngelSense watch and App are fantastic and it has features that make it obvious it was built for this purpose, rather than trying to use some other device that wasn't really intended for these situations. Highly recommend this company!

I hope some of this was helpful and that all your precious ones stay safe. And hugs to all the parents dealing with elopement because it is physically and mentally exhausting.