Sid is an Easter egg on the Halo 2 map Coagulation that is a hidden message much like the "Why Am I Here" message in the Beaver Creek level of Halo 2. Messages like these can be found in many maps, such as Sanctuary. The message in Sanctuary is behind the water. Not "Sid" with the initials CMP and the number '10' in tally marks.
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".