From Halopedia, the Halo wiki
When certain music cues play, or BEATING a level on Halo Combat Evolved Anniversery Edition.
Getting killed by Grunts in firefight on easy, and when my cousin kills me.
I like to perform assasinations, and I think I discovered that there are 7 dead marines on the Halo Reach level Lone Wolf. Also, did anyone notice the grunt face and letters PGGA on the golf club?