Two agents from U.S. Homeland Security’s ICE unit arrived at my door in September looking for a Polish lady — not a person, but a painting: Melchior Geldorp’s “Portrait of a Lady.” She had, they informed me, been looted by the Nazis from the National Museum in Warsaw. Unsure if these gentlemen were indeed who they claimed to be, I didn’t invite them in. But I knew exactly what they were seeking: My partner, David, and I had purchased this very portrait — ostensibly the work of a different artist — a decade earlier from a major auction house in...