I saw the mother and the little girl bringing in laundry and wandered over to the apartment. I introduced myself and they said thank you again. Turns out they (mom and dad-the young guy I saw) have young twins - maybe 12 months old. The whole family was asleep. The little girl woke up and just decided to go outside. The dad said they lock the door now.
Of course, I always lock the door... but now I'm sure they always will! So it sounds like one of those innocent things that just happen when you have kids!