My eldest has escaped twice.
Because of their autism, our house is locked up like fort knox. At the time, we were living in a cottage with a 5 ft wall around it and the front and 8 ft at the back. And the only access - through a 5ft metal gate.
We kept all windows and doors locked, including bathroom and kitchen! and kept the keys on us. We kept the gate chained and padlocked.
I made the mistake of opening the tiny top window in the dining room, thinking he could never get through that!
Went to change my younger son's nappy. Came back - no elder child. Frantic doesn't begin to cover it! I thought there's no way he could be outside, everything's locked. Then saw the chair at the dining room window. No, there's no way. And there's no way he could get out of the garden. He's a toddler and it's a 5ft wall and gate.
My husband found him walking down the main street through the village.
The second time, I have no idea how he even got out! but he got out, scaled the wall again and wandered into the little shop and I got him back over the gate again. We had the police round. The shop (rightly) reported it.
I showed them my big bunch of keys, pointed to the big wall and the padlocked gate and showed them my two autistic children and my one of me. And they went away and we never heard anything more.
So trust me, it can happen, even when you truly think you have done everything possible to prevent it.