once they know about the removal of the mouse-ball, try taking it out, and putting a tiny bit of sellotape on the rollers inside - they'll try the mouse, see the ball is there, and then bang it on the desk a few times before checking the lead, etc.
A guy here at work did an April fools where he put everyone's phone in a clear plastic bag, and sealed it with an elastic band. He then printed a letter on company paper saying "your phone has been disinfected, do not remove the protective bag until you have been instructed that it is safe to do so".
Amazing how many people were answering their phones, saying "sorry about the rustling but my phone is in a bag... what? pardon? I can't hear you.."