Sign up to disable those pesky ads. I know what's behind the door. If you keep going down the hall past the stars office and through the door at the end, you will see a group of zombies feasting on a corpse. If you turn left and run all the way down the hall, you will see a door boarded shut. This is the same door which you are refering to. Need proof? Check your map. They skrewed up with the graphics here, on one side, there is a gaping hole through the bottom of the door, but on the other side, there isn't.
That door connects to the hall outside the library/ aka hall beyond the hall where the STARS office is. Where Leon B sees Sherry run down and crawl through the door's hole. Yeah I was confused too at first, but I found out eventually.
codename: HUNK is right, the zombies won't be there in Claire A as she meets Leon there, but in Leon A there will be zombies there. So if you want to check it out go for Claire A. =P The reason she meets Leon there is that, in Leon B he meets Sherry there, she runs off and under the gap of said door, then he picks up the key she drops (one of the Precinct Keys, pretty sure it's Diamond) and heads back. After which Claire enters and asks Leon if he's seen "a little girl", aka Sherry. In the Leon A/Claire B route this room just has zombies in it and no cutscenes to my recollection. EDIT: On further inspection, after playing Darkside Chronicles some last night, your AI partner will bust down said door in order to chase after Sherry when you first see her. Case and point, I believe. (5:27)