Perhaps a misleading title; I have not found the easter egg, rather I have found a part of it and would like the help of the community to figure out the rest.
I was playing co-op campaign (me and one other person) on heroic with no skulls on the mission Infinity. Just after you obtain a Mantis, you have to clear a few really large corridors of Covenant and Forerunners. In those corridors, after killing most of the enemies so it was safe, we stumbled upon a few interesting sites. In the first corridor, straight in front of where you get the Mantis, at the very end on the left there are 2 consoles that you can interact with; an announcer lady says something for each one. In the corridor perpendicular (the next logical place to go) at the end on the right there are two doors with the ‘press (button) to open.’ You go to the right column outside the doors and walk into it, rather, through it; similar to the consoles the announcer says some more stuff. There was a fifth door after this area, though I don’t remember exactly where, so our idea that you have to have 4 people each pressing a button at the same time was disqualified.
Fly my monkeys, Fly!
How long will it take to figure this one out?