On the level where the Brutes are attacking your barracks, when you walk into the hallway where the Warthog explodes, keep walking right to the end (kill enemies first) and there will be a marine knocking on a door trying to get in. The person on the other side taunts him, asking for a password.

The two are voiced by the same guys who do Church and Caboose in the webseries RedvsBlue.

RedvsBlue is a machinima and used Halo 1 and 2 for their show.