This is quite funny. Go 2 the level crow's nest then go as you normally would
until you reach the big area with all the trucks(where the warthog gets blown 
up by a plasma grenade). Then kill all of the enemies and instead of goin through
the door where the brute comes out of keep ggoing srait. When u get to the end 
turn look to your right and you will see a marine trying to get in a door with 
another marine on the other side of the door asking the marine to give him a 
password. I'll let you find out what he says its helarious.