On the mission Crows Nest when you get to the part where the grunts blow the Warthog up with a plasma grenade, instead of going through the door to your left after you've defeated the grunts, go all the way to the end of the tunnel where there should be a man standing in front of a door to the armory.
If you listen the man in front of the door will be arguing with a man behind the door about if they should let him in or not.