I've heard that, at an air force base where civilians work, there's a blue light that turns on both on the door and inside the room, when that room is in "class

I've heard that, at an air force base where civilians work, there's a blue light that turns on both on the door and inside the room, when that room is in "classified" mode, and they are only allowed to do classified work when the light is on and the door closed.