Just about every printer on the planet leaves a digital footprint – tiny, almost invisible yellow dots on every page they print, featuring information such as the printer’s serial number and a timestamp. To pick up the dots, you’d need a microscope and a blue light.

The information is there for the exact reason you would expect, to make pages traceable. It’s a practice known as digital steganography, which IEEE Xplore define as “the art of inconspicuously hiding data within data”.