i recall a comment by someone claiming to be working in an antarctic base- everyone working outside would be wearing the same suit, but you could subconsciously recognize who you were looking at, at great distances, simply by the gait.
It doesn't even need a radar sensor. Everyone moves in a different way, so that extracting patterns from the phone accelerometer can help to identify a person.
Two people carrying the phone in the same pocket or hand will produce very different patterns that can be analyzed to extract the different behavior. All it needs is one chance in which that pattern is successfully paired with the person's identity, and from that moment on all their activities can be identified.
mmWave can be used to identify unique traits http://www.cs.ox.ac.uk/files/10889/%5BDCOSS19%5DmID.pdf