The Just Judges is the left panel of the polyptych Ghent Altarpiece, which was painted by Jan Van Eyck a few years after his brother Hubert designed it (although there is no easy way to verify this). The altarpiece resides at St. Bavo's Cathedral in Ghent, Belgium. Since its creation in the early 15th century, the altarpiece has been censored, pillaged, forged, nearly destroyed in a fire, coveted by the Nazis, and stolen so many times that it is considered the most stolen artwork in the world.
In 1934, the Just Judges panel was stolen from the cathedral, possibly by a Belgian man named Arsene Goedertier. The Bishop of Ghent received a ransom demand for one million francs to guarantee the safe return of the panel, but the panel was not returned.
On his deathbed, Goedertier claimed to be the only person who knew the location of the panel, and took the secret with him to his grave.
A replica was made in 1945 to put in its place.
There are numerous theories as to the who, what, and why of this theft, but there is no concrete evidence to support any of them. However, investigators are fairly certain the panel was actually hidden in the cathedral itself after it was stolen, but it cannot be proven without a doubt, nor can they determine where it went from there (the cathedral has been searched numerous times, and x-rays have been used to scan 10 feet below the floor). Tips continue to come in, but to this day no one knows where the panel is, and it is assumed by many to have been destroyed years ago.
https://www.theguardian.com/artanddesign/2013/dec/20/ghent-altarpiece-most-stolen-artwork-of-all-time