Mitch is a wanderer, well, was really badly. Luckily he's better now that he is older. I had him brought home to me once when he was about 18 months old, I was making lunch and he just left. Adrien is also like that, but not nearly as bad and usually only does it when the older two are outside. But with Mitch, he'd wander out any chance he could get when he was a toddler. We had to put locks on all doors at the top, the little latch things, and even then he learned he could just push a chair up to it after a month, but luckily hearing a chair being pushed across the floor is a pretty noticeable sound. But, Mitch has never been diagnosed with any disorder, he was just.....a wanderer. Sorry hun
Not your fault at all, it is something you should not feel terrible about, though I understand the feeling. And, as for windows, they have those little twist lock things you just put on the window, like on the side. They look like this
And, off topic, I MISS YOU!!!!!!!!!!!!