Standards of assessing provenance are shifting for those institutions housing pre-eminent permanent collections.  As Maxwell Anderson (formerly director of the Dallas Museum of Art and the Whitney) puts it: "The attitude used to be, don’t acquire something you know to be stolen — that’s a very low standard.  The attitude today is, don’t acquire something unless you know it’s not stolen. It’s a 180-degree difference". 

The Metropolitan Museum of Art in New York is taking this seriously; it intends to hire a team of four staff to scrutinise its gargantuan inventory of more than 1.5 million works.  What a task - I don't envy them.