How To Unlock: Finish the 'Under Cover Of Night' mission without having the guards raise an alarm.
“Under Cover Of Night” is the second mission of the Unfinished Business campaign. You have to get through the mission without having the guards spot you and raise an alarm. Follow the checkpoint markers through the level. Take out the soldiers that you can't sneak past, and reload your last checkpoint if you get spotted. Some people (including me) can't get this trophy to unlock, even though they did everything perfectly right. The only solution to the problem at the moment is to delete your game saves and try again.