Let's see...
A passenger is someone who's being taken for a ride, going along with someone...
I think that's a reference to the mole in the CIA. Still no idea though, although I said it was Dixon or Jack on another thread.
I like the theory about Lauren being Vaughn's passenger when Julia saw them. But then she saw them kiss (on the cheek, I must say), so she could have caled Lauren the "new girlfriend".