DS2 is ASD (2.5) and already an escapologist.
We learnt the hard way that he could climb through the wooden 5 bar gate at the bottom of the garden (he made it onto the road that time, having run down the lane at the back) and we chicken-wired the gate to stop this happening again, and hung a pipe beneath the gate. There are 3, 4 and 5 foot fences round the garden and we replaceor repair panels as they need it. At the same time, I put a bolt on the front gate on the outside of it IYSWIM, which he can't reach from his side. He loves being outside and normally we're in and out of the house, all gates closed, with hiim.
Despite this, he managed to escape from the garden again this weekend, front gate this time. The postman hadn't bolted it and DH hadn't checked to make sure it was bolted. He made it to the road again.
In less than 2 mins, which is what Dh said it took to realise he wasn't still playing by fhe car, run round the garden checking his usual haunts and then make the discovery that the sodding bolt was off.
I'm at my wits'end basically. His escapist potential terrifies me and I've spent a fair bit of time crying this morning because sooner or later, one of us will screw up and DS2 will get out again and end up killing or injuring himself. The river is at the bottom of a meadow that is basically just outside the garden gate. And to think, when we moved here, we did so for the garden and the great outdoors, and how nice it'd be for the boys. 
I want to fit something to the gates that will sound a buzzer in the house when they're opened. Not a burglar alarm, just a buzz so I can check it was just the postman or whatever. Haven't a clue where to start looking. 
Neither DH nor I are negligent, and we're so new to this all, but can see we have to up our game. Escaping is just DS2's Thing.