Hidden Grunt
Warning! This may require you to start the level again. 
On the last level (The Maw) when Echo 419 gets shot down, go down aways until you see a tunnel on your right. Get out of the warthog and go through the tunnel. You should be in a corridor with another corridor to your right. Go there. In this corridor there should be a grunt. He won't shoot you or anything. Go up to him and see what he has to say.