Here is a listing of the Secret Achievements in the game. Note that there are four "Secret Achivements", and including the normal achievements, the total number of achievements is 30.
Secret Finder[]
Find a secret level. 10 Gamerscore
Traitor[]
Kill your teammate 50 times. 5 Gamerscore
Consolation Prize[]
Die enough times and we give you a prize. 10 Gamerscore
Prison Food[]
Get eaten alive. 5 Gamerscore