On the level "The Ark", when you reach the room with sleeping Grunts after destroying the Scarab, if you continue farther in the level there is a Brute taking a "leak" in the corner. Originally, he has his hands in front of him, but after a short time he will move his hands behind his head and then shake the excess urine off himself. After a while he will move into the main hallway and get into position waiting for intruders. If he notices you he will instantly go into attack mode.
It is also argued that he is masturbating.
The best way to see this is to go to Rally Point Bravo, then go straight and then look at the right corner of the room. If you play the whole level, when 343 Guilty Spark stings the curious Marine, go into that room and do what is described just above. It is often advisable to kill the marines who accompany you as they often alert the grunts or brute that means you don't have a chance to watch this brute "take a leak". 300px
Trivia
- If you look at the walls and the floor after he is done you can see what he has left behind (A big wet spot).
- If you watch him for a short while, one of the player's Marines will say "Chief, tap him out".