Hidden Marine
From Halopedia, the Halo wiki
The Unarmed Marine is a hidden Easter Egg on the level 343 Guilty Spark. His character model is that of Private Mendoza, and was simply left behind by Bungie when they were making the game.
To find him, you must first stay inside the Pelican that drops you off at the beginning of the level. You can do this by throwing grenades as soon as the game allows you to do so. This will allow you to stay inside the dropship as it flies away. After it goes a certain distance, the Pelican will drop you off and you will be outside the level's barriers. After exploring around, you will eventually come across the Marine. Upon your approach, he will say either:
"Sir!"
"You scared me sir, and now I've... soiled myself."
or
"Jason Jones is the greatest man I've ever seen!"
He can be shot and killed, just like any other Marine.
