A pencil drawing by Carl Spitzweg, which was auctioned on Wednesday, would not normally not have drawn much attention. After all, the small-format portrait didn’t break any auction records — even though it reached a way higher price than auction house Christie’s initial estimate of €1,000-1,500 ($1,300-1,700), finally fetching €18,750. Its provenance is what makes it so provocative: The work was originally owned by Dr. Henri Hinrichsen, a Jewish music publisher and art collector who was murdered in Auschwitz in 1942. The picture is one of more than 1,500 works from an art trove known as the Cornelius Gurlitt Collection,...