Mission: The Pillar of Autumn
At the start of the mission, jump on the yellow container in the first room, and wait for 15 seconds. Now follow through the level until you speak to Keyes, and he will hand you a pistol. Continue forward, pick up ammo, kill the 3 grunts, and return to Keyes. Shoot him in the head to kill him. Doing this will make 3 invincible troops come out of a secret room that was previously locked. You'll have to lure them away from the door, by sitting in the middle of the room, and then quickly make a dash for the newly opened door without being killed.
Once inside, you can look at the ceiling above and notice the "Megg" Easter Egg using your flashlight!