How could you see what was down the corridor without putting your head out of the door?

One way to see down the corridor without putting your head out of the door is by using a periscope. A periscope consists of a tube with mirrors at each end that allows you to see around obstacles. Alternatively, you could use a camera or smartphone with a live video feed to remotely view the corridor. Another option is to install a fisheye or wide-angle lens on a security camera placed strategically to capture the view down the corridor.