At least one was confirmed to indeed be exist - quite fascinating.

One day hopefully we can find out if the remaining giant looking open area exists too!

Wouldn't there be other ways to detect voids? Strong X-Rays, penetrating radar, or sound wave / seismic measurements taken from every outside and accessible inside space, combined into a 3d model / visualisation. Or is the stone used that impenetrable? I have no idea about these things.

Those other methods do get used, but are of limited utility because the stone is that impenetrable - and it’s a masonry structure with a significant amount of joints, random small voids, etc.

It really does benefit from something like muon/neutron radiation, where absorption is quite low/penetration quite high, so we can ‘see’ pass the first couple centimeters to meters.