I was CTO/head of R&D at a company where we were sensing where someone touched an object based on sounds. I was able to semi-accurately position a finger tap on

I was CTO/head of R&D at a company where we were sensing where someone touched an object based on sounds. I was able to semi-accurately position a finger tap on a glass phone surface. That was in 2016. Caveats: We used contact microphones. Taking the data out of the air from an acoustic signal would be much harder, but not impossible in a quiet room with some fancy DSP. There is a symmetry such that tapping the same distance from corner A is indistinguishable from tapping the equivalent position in corner B. Still, for a 4 x 4 PIN entry that's useful extra info. It depends on the physiology, finger length and nails of the operator, and how they hold the phone. The fact that this may also be a unique identifier of the operator should not be a surprise.