I've been listening to the album all morning while working on a research project - nothing like a little Raiders Theme to get those creative juices flowing!
Anyway, I believe there may be a Russian Theme present in the movie which is not featured fully on the soundtrack. For support of this theory, please listen to the Finale track from 3:55 - 4:52. The theme should be especially evident from 4:46 - 4:52. As we all know, the end credits music gives John Williams the opportunity to recap the major musical themes of the film, and this segment is the only portion of the finale with no definite correlation elsewhere in the score.
I'm thinking that the action music for the Hangar 51 sequence has been purposely omitted from the soundtrack, and may center around this theme. For further reference, the leitmotif can also be heard near the end of Irina's Theme as well as scattered throughout The Spell of the Skull and Ants!
Sounds great to me!