M on top of Coagulation
From Halopedia, the Halo wiki
On the Halo 2 map, Coagulation, there is a large yellow letter M resembling the McDonalds logo that can be viewed in-game. Though it isn't likely a reference to the fast food franchise, the egg is often connected to an existing rumor that there was a McDonalds logo hidden in the level. It is disputed whether this is in fact a third Megg Easter egg or not, but this seems to be up to speculation, and player interpretation.
Walkthrough
First, change your character model to an Elite (this is needed because they get out of the Spectre from the front) and make a gametype where you have some Spectres. Once made, go to the Blue Base in Coagulation. Use the Spectre's boost to climb up the right part of the cliff near the base. There is a ridge on the wall as you continue left that forces you down, so stay clear of it by moving slightly down from it.
By now, you should be completely sideways, and approaching a ridge on the cliff. Keep boosting on it, and you'll eventually get over it. It should take a few tries, but you should stay patient.
When you get to the very top of the cliff, turn around in your Spectre, keep its left wing on the rim you originally approached, move the nose of the craft slightly over the rim, and search for a small notch in the cliff. Then, get out in a way so that you stay on the ledge.
Walk along the cliff towards Red Base but make sure to not walk on the edge. There are notches in it that you can fall into, and never get out of. Once at the back end of Red Base's cliffs, look around for a white patch in the rock. At first, the patch won't look like anything, but back up and look again. There is an M clearly visible on the cliff.
