Fast forward to the part when Bateman is chasing the girl through his building with a chainsaw, almost completely naked. Well, he'd be completely naked if he hadn't put on sneakers between running to the apartment door and running in the hallway...
If you pay attention to his feet during the sex scene right before this he is wearing his sneakers, which shows that he's probably planning on chasing someone. I have the unrated version so I don't know if the sex scene on the rated version or not
I own the uncut version of this movie, so I'm this might help.
Right before Patrick chases 'Christie' down the hall he is in bed having sex with 'Christie' and another girl and he already has his sneakers on. This makes me think that he is planning on chasing someone, but that's just a guess.
I hope I've helped.