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I tried this, and it worked. More interestingly, I found that having a smartphone with a forward-facing camera helps a ton. If you open your camera app, you can push the phone under the door just enough so that the camera is looking straight up at the handle, but you can still see the screen. It serves as an extra eyeball you can stick under the door. And if the room you're trying to access is dark, just turn on flash mode. The screen will brighten more than usual. If you push the phone under the door just barely, the light from the screen will largely bounce off the bottom of the door, and right back into the camera lens, making the image even easier to see.