To find this secret you need to the level:Keyes, then you need to get to the mutated captian, after killing all flood and covenant on the bridge look at the mutated captian and you will see he is made of 2 marines and himself, then shoot him and about a second later you will see a green flash or glow (depending on the weapon you use). The only weapon that doesn't work on this is the needler, I like to use the shotgun and plasma pistol. This also works with dead and living flood. Also, when a Flood is killed the tentacles(like are in the face of the flood infected Elites)pull into the dead flood right as they die.The tentacles are actually the legs of the flod infection.