That's odd, Bert--I didn't get this far before my game started doing strange things to my computer and I quit, but MaGs says in her walkthrough for both ways (Room 507 first or Hellouin's office first) that you get the key by looking closely at the drawer by the bed.
Syd, 507 was the one you lockpicked into (if you chose that route). I think the key was in the endtable to the right of the bed, but I wouldn't be prepared to put any money on it...