Stick an accelerometer on there, and detect spikes corresponding to someone tapping the device. Then implement some sort of rhythm-based security as desired :)

Turning it around would be pretty hard if it's plugged into a PC, and it can't really do the sensing unplugged unless it's got some sort of internal power, but tap-detection should be fairly robust.

You'd probably want to have it remain in whatever secret-mode until unplugged, to stop spurious jolts from accidentally un/re-mounting it (unless you're really paranoid)