Dunno what made me think of this just now. When I worked for IT in a school district way back in the 90s, a librarian told me she kept a supply of mouse balls in her desk because kids would steal them out of the school computers. What I remember about those balls was they picked up dust and crud off surfaces. Pretty soon optical mice came along and they were history.
At one of my jobs a guy ran the speaker wires from the adjoining cubicle in and out of his own computer so he could mix things into the other guy’s audio, mostly music and talk radio, at very low volume so it sounded like random stray signals. Took the guy like a month to figure out what was going on.