[LPisGood]

Indeed, this same principle has been shown to work with sound waves and not just RF waves. There was a paper a few years back that used car speakers and the microphone to be able to detect the number of people in the car for the purpose of detecting children or pets left in hot vehicles.

[umvi]

Generally I think this is called "tomography" (https://en.m.wikipedia.org/wiki/Tomographic_reconstruction) i.e. the reconstruction of higher dimensional data from lower dimensional data. Your brain can do it automatically in a lot of cases. For example if you see the shadow of a rotating cube on the wall, your brain can reconstruct 3d information about the cube even though you only have access to a 2d projection