The Secret Marine on The Covenant is a Marine hidden in a hard-to-access location in the Halo 3 Campaign level 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.