How To Unlock: Collect all secrets.
There are 30 Secrets scattered across the levels of the Single Player Campaign. They are laptop computers with orange screens. Once picked up, they are immediately added to your gamesave, and will not reappear in the world. If you miss one for any reason, you can use the Chapter Select to go back and pick it up. Make sure to keep track of the level though, as there is nowhere in the game menu’s to see which levels are cleared of secrets.