Jenny, feel free to move this post if there is a better location for it.
I came across this article by John M. Kleeberg relating to laws and how courts handle them regarding different interpretation of lost and found property around the United States including treasure troves. It is lengthy, but a very interesting read. I'm not sure how up to date it is.
Lost Treasure.... can it be found? From the article: "Peters hopes the making of the belt may prompt the return of an old one, that of the Wampanoag chief Metacom. It has not been seen since it was taken to England in 1677 following the bloody King Philip’s War between Native Americans and colonists." read more
...if the painting ever surfaces, it would be worth over $850 million in dollars. It would then belong to the Polish nation and be placed back in its original frame which sits on empty display in the National Museum of Krakow.