I was about to mention this in my post but wanted to keep it short :D. The smudge marks are probably as good of an indicator as the heat signature, considering that they don't degrade with time. It also works better with the pattern-type unlock screen on android (but not so well with the pin-type).
That reminds me, you could build a system that records and highlights the smudges (by using a circular arrangement of LEDs and a fixed camera, or by taking pictures from multiple angles under a single light source). It should make a good weekend project, maybe I'll try it :)
That reminds me, you could build a system that records and highlights the smudges (by using a circular arrangement of LEDs and a fixed camera, or by taking pictures from multiple angles under a single light source). It should make a good weekend project, maybe I'll try it :)