For this achievement, you have to detach every detachable security camera in the game by shooting a portal behind them.
Here are all the locations (scroll down a bit): http://www.xbox360achievements.org/forum/showthread.php?t=54...
And the attached video will help as well.
If you happen to miss one along the way, reload a previous save point. If you can't go back and get a camera, sorry, but you'll have to start all over as this must be done in one playthrough.
Credit for the complete camera list goes to both laundryman (Gamertag: Laundry x360a) and xbox360achievements.org.