spoiler

Explained in the film.

*mini spoilers*



*He knew his girlfriend hid the keys because they were hidden in a framed picture of the two of them and he knew he hadn't been the one to hide them. He then flashes back on her telling him earlier in the film (in the elevator) that she had been at a particular apartment on another floor on more than one occasion because they had been getting that tenant's mail by mistake (which was a lie). That explanation/her coming back from that apartment seemed to have struck him momentarily as 'odd' when she told him that -- so he goes to that apartment to try the keys in that door.


*On the ceiling was a 'pentacle'. Maya earlier in the film tried to tell him to look for a pentacle as one of the stages of preparation for his becoming the vessel for the anti-Christ; she tells him that it's probably around his bed somewhere. This pentacle was on the ceiling directly under where his bed was, one floor up.




"I will not go gently onto a shelf, degutted, to become a non-book." ~ Bradbury