Hidden Marine

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The Unarmed Marine is a hidden Easter Egg on the level 343 Guilty Spark. His character model is that of Private Mendoza. It appears that Bungie wanted his model to load before the cutscene that he later appears in; they thus coded him to load early in the level, and simply used the area outside of the level as a storage area. He can be shot and killed, just like any other Marine.

How To
To find him, you must stay inside the Pelican that drops you off at the beginning of the level; to do this, start throwing grenades as soon as you are able to. Stay inside the dropship as it flies off. After it travels a certain distance, the Pelican will drop you off outside of the level barriers. By 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.

Possible Explanation
It seems that Private Mendoza only appears twice in the level: once during the Jenkins video, and later in the game as a Combat Form. It is possible that Bungie wanted to speed up the loading of the Jenkins video by loading Mendoza's model in advance, but to do so, they needed to put him somewhere. They likely just chose a location that they didn't expect the player to reach: outside of the level.

If that's the case, then this is a glitch, not an Easter Egg -- by definition an Easter Egg is meant to be found by someone, whereas a glitch is completely unintentional.

Trivia

 * A rare occurrence happens when the Marine is found 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.