Hidden Marine

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. He can be shot and killed, just like any other Marine.

How To
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 travels a certain distance, the Pelican will drop you off and you will be outside the level barriers. After exploring around, you will eventually come across the Marine.

Sometimes, when you're inside the structure, bodies will spawn all around where you came in from. Sometimes (if not always), if you surface just before the Jenkins Cutscene, bodies will spawn all around and the Unarmed Marine's body will be where you fought the first Covenant on the level. If somehow you manage to get to the higher level after doing the trick above, Flood Combat form corpses will be around the spot where the Unarmed Marine is standing.

Trivia

 * A rare occurrence happens when played on Co-op Mode, when you hit the first lone Marine while as the 1st player, it also registers as a hit to the 2nd player. If you kill the marine, you also kill the 2nd player.