An 80 year old art theft, as yet unsolved:
"The panel was removed from the frame, apparently with care, leaving the other panels undamaged. In the empty space was left a note, written in French, with the words, "Taken from Germany by the Treaty of Versaile", a reference to the fact that the altarpiece had been returned to Ghent only a decade earlier after having been moved to Berlin during World War I."
The thief was later concluded to be Arsène Goedertier, who revealed on his deathbed to his lawyer that he was the only one who knew where the masterpiece was hidden, and that he would take the secret to his grave. Telling only his lawyer, Georges de Vos, that "I alone know where the Mystic Lamb is. The information is in the drawer on the right of my writing table, in an envelope marked 'mutualité.'"
Despite cryptic clues and numerous searches the panel remains lost, you can read more at the Wikipedia: The Just Judges.