The field that has the best  funding for developing methods to find
hidden objects is medicine.  I'd call a radiology dept of a med school.
Ultrasound might be able to find the hole in the stone.  X-rays or such
would require access to both sides and I assume that's difficult with a
corner stone.  You might also contact an archaeologist,  they do have
some sort of 'radar' which may also be ultrasound.
   Good luck,
   Dan Freidus
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