Half-naked guy

In the final level "Halo", in Halo 3 there is a secret area for the 7th Marathon Terminal (see Marathon Terminals Page). Aside from the Terminal's hallway, there are two rooms: one bright room with a vertical energy beam in the center, and a dark room with a horizontal energy beam and lethal chasm.

It's hard to tell by standing at the edge of the chasm, but there is a small nook on the left side of the room (left, as if one looks out over the chasm). By watching a save film, one can detach the camera and move in there. You can also reach the area by propelling yourself with a rocket or grenade jump to get to the support piece, and then crouch jumping to the next one, then you just turn your flashlight on, zoom in, and you can see him.

In the far corner of the nook is an eerie floating cardboard cutout of Jason Jones, the co-founder of Bungie Studios. He is facing the wall, with his hands behind his head.

Other Jason Jones Appearances
Ironically, there is a "missing person" poster on the levels "The Storm" and "Floodgate" with a picture of Jason Jones on it with its description saying "Khaki shorts and long socks".

Jason Jones is also featured amongst a group of Grunts charging towards the room where the Prophets are broadcasting in a cameo in the cutscene at the beginning of the Halo 2 Level, "Gravemind". He only appears there, half naked as always, on Legendary Difficulty. See this for more.

Finally, on the map Standoff, if one zooms in with a sniper rifle or uses the pancam glitch, you can see a cardboard cutout of Jason Jones on the satellite towers.

Walkthrough
Please note that this is a co-op walkthrough only. Begin the level (on any difficulty) and proceed to the room that contains the Terminal. When you get to the Terminal, turn around and you should see an abyss with a beam at the bottom. Be careful, because the next part is easier said than done.

Have one partner stand right at the end of the floor before the abyss. Have the other partner stand on his shoulders. Then, have the bottom partner jump into the abyss. As the bottom partner jumps, the other partner should jump off of his shoulders, and should land on a small piece of floor. Stand to one side, as the partner will respawn right on the small beam that you are standing on. You can then crouch-jump to another beam, and if you turn left and look toward the corner of the nook, turn on your flashlight. You should see the Lonely Soul.

One other way to get to The Lonely Soul (also easier said than done) is have on person stand just out of the doorway facing the abyss with his Rocket Launcher or the Fuel Rod Gun. Next, have the other person run towards the ledge and jump at the edge. As he does that the person with the Rocket Launcher should shoot at the other person's feet, denoting an exploding jump.