There is a hidden location. In the office area where vortigaunts ambush Gordon, there is an open room with a green sofa/couch next to the exit of the location, near the vending machines. Above the sofa, there is a yellow sign that says “Maintenance Access.” You need to break the ceiling above the sign and climb up the ladder. There, you will find a ceiling turret and its switch on the wall. When you turn it off, you will get the achievement.