Special dialogue Grunt
From Halopedia, the Halo wiki
There are two instances of Grunts that have special dialogue that are found in the form of easter eggs. The first is on the final level of Halo: Combat Evolved. When you are escaping the Installation in the Warthog, you will come to a large jump. If you exit your Warthog and walk to the end of a nearby tunnel, you will see and hear the Thirsty Grunt. The second is in the Halo 2 level Uprising. There is a live Grunt hiding in a corner near two Grunt corpses, known as the Cowardly Grunt. Neither of these grunts will (or can) fight you, and will die like normal Grunts if you shoot them.
