Find and scan all five databank items in a level to unlock the secret for that level. The final secret of the game is unlocked when all prior secrets have been found.
Unlock Gray Alien in multi-player mode:
Successfully complete campaign mode under the hard difficulty setting.
Before you go to were they faked the moon landing there is a wall of pipes just before the elevator room. There is mutegen and health with alien grenades.
(supplied by: wiki)