Halo: The Master Chief Collection - Halo 2 - Why Am I Here? - Halo 2: View this hidden message on the classic Beaver Creek map. This achievement is fairly easy to unlock.
On the map Beaver Creek, as soon as you spawn get to the center of the map where the stone arch is. Looking at the wall of the map in the direction of the base of the arch is head left. Walk up the dirt path through a small opening and jump up to where there are a couple rocks.
On the other side of the rocks on the wall is a drawing you need to look at. Look at it and your achievement should pop! Enjoy!