In "The Hive," after opening a couple alien doors, you will encounter a room with tons of spore-looking things near the doorway. Take a left, then turn on your Duke Vision and crouch, under the branch like things. Then, at the end of the cave you ducked through, there is a dead EDF marine. Walk up to him to have Duke say "That's one dead space marine." His helmet looks a lot like Isaac Clark's helmet from Dead Space.