This particular method is triangulating background aircraft doppler sounds, getting their approximate speed and distance from the doppler shape and looking it up in air traffic logs to find valid matches. it needs multiple data points though.

it's not perfect of course, there are plenty of light aircraft that dont have accurate tracks available, and it would be easy to spoof or mask, but it's very cool bit of work that could be added to the toolkit for people that really want to use it.