Queue the dramatic music because I have a theory... And no its not bunnies (for buffy fans ;) its about phone guys death. All of this was figured out by a friend who doesn't go on the wiki so thank him and if you want more theories, these will be done with either of us or both of us. So here it is:
1: We can easily eliminate Foxy because he is banging on the door meaning he is outside the office. 2: Freddy is questionable as he only plays the tretador march when he is in the office (via power is out) or when he is in the kitchen. I will hopefully get back with more information about this. 3: Bonnie also could not have been in the office because if the west door is closed, then Bonnie can't get in. 4: Chica is the most likely because of the groaning, meaning Bonnie or Chica was in the office and since the west door seems to have been shut, it would have to be Chica.
So yeah, I'm leaning toward Chica versus Freddy but tell me what you guys or gals think in the comments! Thanks and bye!