This takes place on Mission 4: Fort Rifugio
Quite easy to do this if you have the right path, and are playing on Cadet difficulty. Just make your way through the middle of the first area avoiding enemy contact, and head up the ladder that leads into the base on the right hand side.
From there, make your way to the informant to get the achievement. The key is not needed for the achievement to unlock, but is needed to complete the level.
The achievement takes 3 minutes, and the rest of the video is on how to beat the rest of the level without being seen in order to get the "Hidden and dangerous" achievement: