HaLO 3 ?!?!??!?

I found a skull in HALO 3 that apparently doesn’t regitar with the game at all. It’s in the last level, after rescuing Cortana. When you have to go back and destroy the reactor. In the big open room where you do this, it’s possible with someone’s help to get up on the upper level that runs around the whole room. Up there you will find a skull. But, it doesn’t show up on the skull activation screen. And, you got no achievements for it. What gives?

Question posted courtesy of: Halo 3 Special Edition