When you reach the loading hangar where you must defeat the first Covenant boarding party, make sure that you save the lives of all the Marines with you. When every last Covenant soldier is dead, look at your accompanying Marines' faces. One in this room will be cut, bruised, and covered in blood. This is the only place in Halo 2 where such a damaged Marine can be found. You can also see a bloody UNSC Marine in The Art of Halo on page 14.