At that age you can't turn your back on a kid even for a second.
Truth. They're fast, turn on a dime and give you 11 cents change, and are sneaky, curious, and far more capable than they let on. They are busy watching what the people around them do, and can learn by doing so, especially how to undo locks and latches. And they can be in serious trouble before you know they are gone.
Sorry they didn't get him back alive. Prayers up for the family.