This is the "Official" Easter Egg Guide. Please list your finds, guess' and searches in here.
Spoiler Alert
Hey guys, I've just recently had Halo: Anniversary posted to my house. I was on firefight on Installation 04 and I couldn't help but notice a wierd yellow dot on my radar, I knew my marines were dead, my teammates had left the game... What could it be? I figured that map is a replica of the level Halo, so I went into the little interior of the bunker next to the cliff edge (near where the marines are picked up on the mission) and looked through the door. IT'S 343 GUILTY SPARK OMFGWTFSOS
So yeah, if you go into that bunker and look through the doors (the doors are closed) you randomly see the little guy fly up and down the staircase. Easter Egg found ;D