Day 3: 12:00 P.M.-1:00 P.M. - S3-E24
Continuity mistake: In the teachers' lounge Jack tells Chase that the second floor is cleared. When he doesn't get an answer he realises something is wrong and runs down to the floor Chase is searching. After he's cut Chase's hand off he runs out into the hallway and into the lounge which is suddenly on the same floor Chase was on.
Chosen answer: Nobody knew, not even the writers, until at least halfway through the season when they were writing the end. They wanted to keep all options open.
Nick N.