First, go to the security office and flick the lights off. Then, crouch down and go in the corner near the door. When the guard enters to turn the lights on, knock him out. Move his body away, then flick the lights on and off. Hide in the same place. Wait for awhile and another guard will appear to turn on the lights. Do the same to him as done to the previous guard. Flick the lights a third time and the last guard will appear. You have to be careful with him, as he can see you in the corner. Knock him out quickly.