Not a glitch but a blooper, on the second mission when you crash land, there is a bunch of weapons on the map, hidden. Plus an elite and a grunt that crawl to there death. But that's not the blooper or Easter egg here, let alone there is a sniper rifle here some where
but lets get on track, before you go down that narrow cavern go to your left and up a scarab it's heading back to the way you came. There is an elite action button of some sort there, try and make out what that thing says then come back to me.
good luck.