Load up a custom game match for halo 2 pick Coagulation.
1.Head to the 3 rocks in front of red base.
2. Go in the center of the 3 rocks.
3. Look at the rock that the shotgun is pointing at. (It looks like Jabba the Hutt.)
Also achievement "Sid Graffiti" is on same map.
1.Head east of Red base where the two trees and teleporter are.
2. Head into the cave and go straight to the edge where the water is but don't drop.
3. Look at the wall on the left, you can faintly see the words "Not Sid Was Here."