In the Halo 2 map Coagulation, there is a hidden message that says, "Not Sid Was Here" with the initials CMP and the number 10 in tally marks.
Directions[]
Go into the cave systems above where the Overshield/Active camouflage 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." If you look further to the edge the word, "Not" can be seen, making the message "Not Sid Was Here Not."
Video Directions[]