Yes, I understand that sometimes this happens but many of theSome are handled rather cladistine and the original owners agree a "finder's reward" to get their items back without lengthly legal procedures.
original owners are deceased and it is up to the heirs to
supply proof of original ownership. The one case I personally
know of could not after contact with auction house gain their
art work and it was sold. This was over ten years ago but I think
now there maybe other alternatives?
Annelie