The recent case of Golan v. Holder held that Congress has the constitutional power to revive the copyright on foreign works that had fallen into the U.S. public domain as part of its treaty-based efforts to promote the arts through reciprocal arrangements with foreign countries. It further held that such "re-copyrighting" doesn't violate the first amendment rights of persons who had relied on the prior public domain status of the works that had been removed from public-domain status.
The Golan case is, to me, a really absurd outcome and reflects a weird mix of liberals on the Court (e.g., Ruth Bader Ginsburg, who wrote the opinion) upholding sharp encroachments on free speech and conservatives (who joined in a 6-2 ruling) upholding rather large assertions of constitutional legislative power where a strict construction of the constitutional language ("limited times," etc.) would seem to point in the opposite direction. I think it is a bad decision. But it by no means empowers any private party to try to claim that anything in the public domain somehow shifts back to copyright status by virtue of what that private party does. The case is irrelevant to that issue.
As anybody who takes the time to read the ruling will see, the Supreme Court ruled that Congress has the power to make laws that retroactively extend the copyright of certain works that had fallen into the public domain, and specifically said that Congress was empowered to do so if a treaty required it. I do think it's a bad ruling, but it is a far cry from a private party being able to swoop in and snatch works out of the public domain.
It was only a few days ago.
The works in question were still under copyright in other countries.