Gotta admit that's what I clicked through hoping for.

I've seen a few projects that use SDR to detect doppler shifts of reflections of broadcast FM radio station off planes as "passive radar". But that doesn't get much in the way of direction info, just approach velocity. I've pondered building something that uses that and correlates it with an ADS-B receiver, to see it it can detect aircraft that are not transmitting ADS-B. But that'd get me on another "list" if I published it I suspect...

This is not the project I remember, but it's the same idea and technique: https://github.com/Max-Manning/passiveRadar