The sign can be found during the Warthog run near the end of the level. Normally, the player has to jump with the Warthog to cross a big gap. During this jump the player has to aim a little to the left. Done right, the Warthog lands on a platform beneath the normal platform. Here is a small tunnel with the word "Rex" written in blood on the wall. Looking for several seconds at the word unlocks this achievement.