1.) Marathon symbol - go to the level "the covenant" and when you are at the elevevators that take you to the barrier controls, look at the "hold rb" thing you will see in a holo outline the Marathon symbol.

2.) reference to the 5th level in Halo 1 - read the description on mission start on the final level in halo 3 it will say "assault on the control room" this was the name of the 5th level on Halo 1.

3.) secret message in the vines - go to forge on guardian and get out of the level, zoom in on the vine and it will say "blarg" - an rvb reference.

4.) image on plasma grenade - throw a plasma grenade; go to the theatre lobby and look at the pg it has a hippo head which is a reference to Wideload Games.