See, i don't think the passenger has to do with rambaldi. They really tied in a whole crapload of rambaldi stuff in the episode and adding more would make many people's heads explode.
When I heard a first theory of Lauren being the passenger, I liked it. I even started making ideas up in my head. So here is another one of my theories (prepare yourself).
When Lazarey was captured by Sark, and we're talking after he was tortured and after Sark went to Graz and Freplica "died" (i'm still unsure of whether she's dead or not). Sark went to Patagonia and brought along Daddy dearest. While in transport, Sark probably called or talked about Lauren, but refered to her as the Passenger, with that being her codename. He must have also said that there was a mole in the CIA, because Lauren was leaking them information. Lazarey heard this, but thought nothing of it. When Syd talked to him, though, he thought it'd be good to tell her because he knew she was CIA. Then, of course, Lauren shot him, thus protecting her secret of being the Passenger.