S everal months ago the Dutch art detective Arthur Brand was amazed to be contacted by a man who had recently made an uncomfortable discovery about his family’s wartime past: that he was a descendant of Hendrik Seyffardt, a Waffen-SS general and one of the highest-ranking Dutch collaborators. Hendrik Seyffardt (third from left) with other members of the WA Volunteer Regiment in The Hague, 11 October 1941. Photograph: Shawshots/Alamy Not only that, said the man, but he had found out something else: a painting by the Dutch artist Toon Kelder, which had been looted by the Nazis from the famed collection of the Jewish art dealer Jacques Goudstikker, was still in the possession of the Seyffardt family. Dutch Jewish art dealer Jacques Goudstikker, in the 1930s. He died in 1940 while fleeing the Netherlands. His vast collection of art was subsequently looted by the Nazis. Photograph: AFP/Getty Images Kelder’s Portrait of a Young Girl was hanging in the hall of his relative’s house near Utrecht, he told Brand.…