Fun fact: the heat measured is just infrared light, which tell you the objects temperature with the principles of Black Body Radiation, so if an object is reflective, it most likely also reflects infrared and you will see the reflected objects temperature.

If your first thought was “wait, infrared like tv remotes?” Yes! Point the tv remote at your camera to see how “hot” it gets (it’s not hot but it’s sending a lot of infrared through other means)