From Halopedia, the Halo wiki
The Cowardly Grunt is a Special Dialogue Grunt that can be found in Halo 2's level Uprising. The Grunt is hiding in the corner of the first room after the cavern, with two dead Grunts. It will not follow the player and will not fight enemies even when forced by the player. All Brutes in the room must be killed for the easter egg to work.