Enter Hold 1 or 2 on the ship (right after the first boss fight...the Hold rooms are where the Marines are listening to the Commander give his speech). Once inside of either one of the first 2 Hold rooms, creep past the Marines, and look back at them from the north end of the room, so you can see their faces. In each Hold room (or at least one of them), you should spot a few Marines without any pants on (don't worry, they still have some white boxer shorts, though).
Not really a trick, but a cool Easter Egg nonetheless. =)