Secret Marine on The Covenant
The Marine only appears on single-player Local Play, and can be accessed using a Ghost or a Prowler. Sometimes, two or more Marines will appear. It is suspected that the faster you get to the Marine, the more Marines you will find. There appears to be a kind of "time limit", after which the Marine will not be present -- whether he disappears or heads to another location is unknown. Once you encounter him, though, he will never leave his position.
Start at Rally Point Bravo. Grab a Mongoose and drive straight ahead at full speed. When you get to the part where a Marine says, "Citadel in sight". Get off of your Mongoose and grab a Ghost as soon as possible. Drive the Ghost ahead as normal. When you approach a tunnel, turn right and climb straight up the mountain. There, you will find the Marine(s).
You can also do this in a Prowler - steal one from the Brutes in the area before the tunnel. Once you get to the ridge on the top, you will need to pick your path carefully -- some parts of the final ascent are too steep for a Prowler. Also, the Prowler is very slow, so you may reach the top only to find that the Marine has vanished.
- The Marine(s) will usually change faces every time you go up the mountain.
- It should also be noted that the weapon the Marine carries will vary. Usually, the Marine(s) carry Assault Rifles, but they can also wield Rocket Launchers or Pistols.
- The marines may also be there as placeholders for reinforcements that will replace the warthog's crew when it's destroyed.
- The quicker you get up the more marines there are.