Alien Mulitplayer Skin:
Beat Campaign Mode on hard difficulty.

Hidden Warp:
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.

Hidden Tapes:
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.

Alien Grenade:
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.