Don't feel bad. It happens.
If it makes you feel better, ds1 has escaped twice!
I was changing ds2 (also autistic, didn't come out of nappies until 5) and I realised ds1 was gone. Looked everywhere and was really panicking, when dh pulled up (he had been out) I rushed to him, totally hysterical, and ds1 was in the car!!! dh had seen him walking down the street and screeched to a halt in the middle of the street!
Second time, I saw him outside the gate! I got him and about half an hour later the police came and said they'd been called by the people at the shop! Ds1 had gone in there, then come back home
The thing is - Our house was fort knox! Windows locked, external doors locked, locks on kitchen and bathroom doors... me and dh jangled like a pair of jailers!
We saw him a third time, he pulled the chair up to the window sill and tried to climb out of the tiny top window - not locked because we stupidly thought they couldn't get through it!
But even so, outside was a 5ft wall and a double gate locked with chain and padlock!!
So those times he had escaped - he had climbed out of a tiny top window, and scaled a 5ft wall!
It is hard to watch 2! Both of mine have autism and you can be having to deal with something with one of them and the other will take the opportunity!
We had locks on everything, a wall, padlocked gate and everything and he STILL got out!
I too felt like shit. But you have to move on. See if there's a lesson you can learn - in my case it was lock the top window too! But don't beat yourself up.