In the second level (i think) there is an easter egg I found. When you are walking in that tunnel highway place where some of those flying things fly around (lol) than there is a spot that you turn left into a hallway, but if you keep going straight, you will reach a marine knocking on a door and trying to get the guy on the other side to open it. It's pretty funny and when I play the game later today when I get my 360 and halo 3 I'll be sure to be more descriptive. If anyone else has seen this, please feel free to decribe it better than me so others can see it.