I have multiple children. Trust me. Keeping track of all of them at all times is virtually impossible. They wander off when you are by necessity concentrating on other important matters. When I was a toddler and crawled behind my parent's china cabinet and feel asleep, my parents didn't notice until three hours later when they immediately called the police. I still hear about that little incident at family gatherings. Not attempting to exonerate this man, I can at least understand how it could happen. My wife and I made the decision long ago that I would work while she took care of the children even if it meant that we would hurt financially. There have been many a morning that I have gone to work without seeing any of the scenery along the way worrying about how we would make ends meet. One time I remember taking all four kids to the store to grocery shop. When we got home, our four year old had fallen asleep in his car seat. In the rush to get the groceries out of the hot car, we left our youngest in his car seat and didn't realize it for almost an hour. Sammy was fine but it gave us a real scare. Some of the stories that you read about this sort of horrible event have no excuse. Some are just too tragic to contemplate accidents. From the sound of it, I suspect the latter.
I certainly agree it was an accident. Maybe mine always made so much noise I couldn't forget them!