This isn't so much a code. It's just something that shouldn't be where it is. In the level "Guilty Spark 343", there is a crashed Covenant dropship just around one of the corners you must turn (the second or third, I believe). Up above this dropship is a tree jutting out over a ledge. On this object is a Flood-infested Elite (the bulkier Flood ones that don't pop). It appears as you come around the corner and runs back along the log after a few seconds then disappears. It is named 'The Yeti' because it diappears whenever seen. This is really weird because the Flood have yet to be introduced at this point. Also, the Yeti is invincible. You can sneak up on it from behind (which is too long and tedious a subject to go into), but don't have your co-op partner go around the partner until you're in position. It can withstand a pistol whip to the back of the head and a plasma grednade, so we just gave up. It's really fun, though. I suggest you try it.