Alien Mulitplayer Skin:
Beat Campaign Mode on hard difficulty.
In "The Grays" level, go into the fourth room where you can see the monorail out the window. Use the computer there and a message "Lift Activated" will appear. Then go outside to the monorail and stand on it. Run to the back on the monorail and you will be warped to a new room.
Scan the five databank items in a level to get that level's secret. The last level's secret will be unlocked when every secret of the previous levels has been obtained.
In the Life or Death level, search the dead soldier in the wrecked elevator where Cole starts the level to find a small key. Use it to open all the lockers in the level. One of them in the first power conduit pit has a databank item to upgrade your grenades to track their target.
Thanks to Revolution readers adam east, raynor, and Nuri!