[m2fkxy]

Yes, radar shadow is how you would find it. But your sensor must be performing well enough for it to be able to distinguish a shadow from the surface backscatter (ie. radar signature) the aircraft is sitting on. This is usually not a problem for rough surfaces (eg. grass or dirt, or some types of pavement), but it can be more problematic for surfaces with more specular scattering.

[KeplerBoy]

For those less obsessed with SAR images: calm water surfaces are good examples of surfaces with specular scattering, they are basically black/extremely low magnitude areas in SAR images. Example: https://x.com/umbraspace/status/1831111648498810967

I wonder if the tarmac/runways used for stealth planes take this into account and are somehow especially smooth or otherwise special. Also how would such an image look like, I guess one would still see some multi-bounces between aircraft and tarmac making it's way back to the SAR antenna.

[m2fkxy]

  For those less obsessed with SAR images

Touché!

Also good point regarding multibounces and multipath. I would expect eg. landing gear returns to be standing out in those cases (cockpit too, although probably the canopy is coated to prevent radar penetrating).