It would certainly show up. Maybe not as a recognizable plane, but at the very least you could make out the plane-shaped shadow (or more accurately lack of reflection).
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.
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.
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).