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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