Gameplay

Sid Graffiti

From Halopedia, the Halo wiki

Sid Graffiti is an achievement in Halo: The Master Chief Collection. This achievement is obtained by spotting the "Not Sid" graffiti on the walls of Coagulation in classic multiplayer.

The hidden message can be found in the watery cave, on the upper level looking out over the pool of water on the lower level. The achievement is worth 5 Gamerscore.