Almost all color printers nowadays have a hidden watermark embedded in everything they print that tells investigstors the make and model of the printer something was printed from, along with the time when it was printed out.
I'm not making this up.
Edit: I should have scrolled down to see other people discussing this exact thing.
You shluld absolutely not trust browser extensions that can track your viewing habits. Its a 99% chance to be actual spyware, even if it does provide the service it claims.