I'm honestly not getting at you worry, but I have a brother with PMLD - exactly the sort of child you'd expect never to be out of your sight...
but it happened, occasionally, that we all as a family had lapses of concentration. He once got out of the back door and ran away (he was about 10), and all of us were out searching for him for ages. God knows what would have happened if there'd been something like a railway crossing in the village as he has absolutely no sense of danger and wouldn't respond to anyone shouting his name.
Or the time we lost him in the supermarket (aged around 12) and ran up and down the aisles and out into the carpark for a heart-stopping few minutes until he was located.
With the best of intentions, it happens. And yes, FWIW I'm sure my DM spent days having nightmares and being paralysed with guilt that she was the worst mother in the world.....