When did the mother left the building?
He is using the only elevator available and then he ask the janitor to stay at the door. If you have left during the time Sebastian is going door to door, the janitor would have told her what was going on...
The other plot on the history is... who keep the kids in the 4th "A"? did they get them? if not, how did they scape? When Sebastian arrives to the appartment, the policemen at the door say "no news here".. ??? they don´t know the kids where in the 4th floor?. Is the policeman in the 3rd floor involved?
I think the end is poor...