Three dead Grunts
They can be found at the beginning of the level Assault On The Control Room if you jump all the way down to the bottom of the pit. Since you will die from such a high jump, you have to jump back in the pelican or use mods to view this egg. The egg consists of three Grunts (two Grunt Minors and one Grunt Major), laying together in their dead state, with most of their geometry overlapping. It should be noted, that the Grunt Minors are "Curlback" Grunts.
If one looks at the relevant scripts in HEK (Halo Editing Kit), one will see that these Grunts are the same ones that appear in the beginning cutscene: They are killed and teleported down there. This being said, it is not a genuine Easter Egg.
You can also view the Easter egg by starting the level with two players and having one player exit the pelican while in free-fall and flip it, but it is tricky to pull off.