Halo: The Master Chief Collection - Halo 2 - Sid Graffiti - Halo 2: Spot the Sid graffiti on the walls of Coagulation in classic multiplayer. This achievement is fairly easy to unlock.
On the map Coagulation, as soon as you spawn in on the map, find the red base. Head over to where there are three trees and enter the cave nearby.
Once inside head a little further inside and look on the left and zoom into the text that is on the wall.
Look at it and your achievement should pop! Enjoy!