Sid

On the Halo 2 map Coagulation there is a hidden message much like the "Why Am I Here" message in the Beaver Creek level of Halo:Combat Evolved.

Directions
Go into the cave systems above where the Overshield/Active Camoflauge spawns, and in the arch/window look to your left and you will see a message in the rock that says "Not Sid Was Here".