This one is in Halo 2 on the multiplayer Containment (Level), where a face can be seen in the Halo ring. The face is thought to be Bungie team member Frank O'Connor or Chris Carney.
It can be found by standing near the red team's base where the turret is, then zooming in with a sniper rifle at the ring (Even though the Sniper Rifle Scope isn't required, it may be seen through regular zoom). This guy is the creator of the Halo series and has references throughout the game such as on:
Beaver Creek
Beaver Creek (Level), if you go up by the sniper rifle and then further some there is a rock hit the rock off the ledge, there should be words, these words make a reference to him "why am I here" .
Coagulation (Level)
Coagulation (Level) if you go into the teleporter at the red base and go into the tunnel there should be a ledge and a path down to the over shield go to the ledge and look at the wall next to the ledge to the left there should be some more words with a reference to him.