Dancing Marty

At a big open location when you are in a banshee fighting AA-wraiths and blowing up spores, there is a dark section you can travel to off to the side of the map with a ramp to the left that you can climb up. If you go to the top of the ramp and then jump down to the adjacent walk-way a dancing Marty appears. Mary O'Donnell is the man behind the music in Halo and a nice little music track is played too.