Find the hidden Rex sign on The Maw
Halo Combat Evolved THIRSTY GRUNT – Achievement Guide
Complete Skull and Terminal CHECKLIST(Halo 1-4 / Google Docs)
After you passed the bridge during your escape on the warthog you will have to drive a little further. Once you’re about 600m from your target you will come to a big gap that you have to jump. Aim slightly to the left. You should land next to the ramp. Use the Warthog to get over the wall and walk into a small room that has REX spelled out in blood.