Hidden soldier in Cairo Station

The colloquially-termed "Hidden soldier in Cairo Station" is an unexpected marine in Halo 2 the campaign level Cairo Station. At the Fire Control Station aboard Cairo Station, where the Covenant Bomb is, there is a secluded Marine. Right when you exit the elevator to the Fire Control Station, take a left. Then take another till you are at the very back of the room. There you will see a fellow UNSC soldier holding an SMG. You can keep pushing him by walking into him. If you push him close enough to the covenant enemy he will join you in combat.

Pushing your Marines into battle also works on other levels, if they're less than enthusiastic about fighting.