The Portrait of Dr. Gachet hasn't been seen publicly for decades
It's not so much a whodunnit as a where-is-it, the "it" being a lost masterpiece of Vincent van Gogh. As the New York Times explains, the mystery revolves around The Portrait of Dr. Gachet, which van Gogh painted in 1890, just weeks before taking his own life. The portrait of a real-life doctor hung in two public museums -- the Stadel in Germany and the Metropolitan Museum of Art in New York -- throughout most of the 20th century before being sold at auction in 1990 to a Japanese buyer. It hasn't been seen in public since. The story tracks the efforts of art sleuths who specialize in just this kind of discreet detective work and presents a best-guess scenario: The painting is likely in Switzerland, in the hands of a very rich and very private family.