Urinating Brute

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.

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. 

Trivia

 * The Brute is not always the same rank, however it's almost always a Captain.
 * Usually if the Brute is a higher rank it will notice you a lot faster.
 * If you watch the Brute from the wall where he is urinating you see that nothing is there. This was likely overlooked during the development process of the game.
 * This is possibly the only Brute-related Easter Egg in the Halo trilogy.
 * In some cases, the Brute will turn around before you enter the room, even if you get rid of all soldiers and leave the Grunts alone.
 * If you watch the Brute on Theater mode, he seems to be "zipping" or "pulling" up his pants before turning around.
 * You can kill all the grunts in the area and make a lot of noise and the brute will not see you.