The Easter Egg appears in a cutscene in Gravemind. To see it, you must start the level on Legendary. During the beginning cutscene, after the Gravemind finishes his long conversation with the Arbiter and John-117, the camera fades to High Charity. In High Charity, there are Brutes defending the room where the Prophets are hiding from the Elites. When the Grunt climbs the arch and the camera focuses to the crowd up ahead, you can see Jason Jones standing in the crowd of Grunts.
In Halo, there is a secret area for the seventh Terminal. Next to 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 a 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, if one looks out over the chasm). When watching a saved film in Theater, 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, crouch jumping to the next one, turning the flashlight on, and zooming in.
In the far corner of the nook is an eerie floating cardboard cut-out of Jason Jones, the co-founder of Bungie Studios. He is looking at the floor, with his hands behind his head.
Please note that this is a co-op walkthrough only. Start 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 (Person A) stand right at the edge of the abyss. Have the other partner (Person B) stand on his shoulders. Then, have Person A jump into the abyss. As Person A jumps, Person B should jump off of his shoulders, and land on a small piece of floor. Person B should stand to one side, as Person A will respawn right on the small beam that B is standing on. The two can then crouch-jump to another beam, and if they turn left, look toward the corner of the nook, and turn on their flashlight, they should see the Lonely Soul.
Another way to get to the Lonely Soul, also easier said than done, is to have Person A stand just out of the doorway facing the abyss with his Rocket launcher or the Fuel Rod Gun. Next, have Person B run towards the ledge and jump at the edge. As B does that, Person A should shoot at Person B's feet, with the resulting explosion propelling B to their destination.