Mr Oswald claimed that, just before he left the building, a credential-carrying crewcut man came up to him looking for a telephone, and Mr Oswald pointed him the way to the nearest one.
If Mr Oswald was telling the truth about this (and Mr Pierce Allman seems to confirm it), then we have Mr Oswald at the front entrance shortly after the shooting apparently noticed by not a single co-worker. And why would anyone take particular notice of him? He was just another manual worker.