how does that work if the case is never opened?

You can connect multiple switches in series. Eg one could be a reed switch that detects whether the PC is within 1mm of a tiny magnet in the floor.

This is a good idea! Kind of makes me wish I was a drug kingpin with a PC full of secrets so that I could justify trying that. Sadly I would just end up with a kid rebooting the PC by bumping into it :D