Although I never knew about the Mystic Lamb or the mystery of the Mystic Lamb I do know that this article by Patrick Bernauw is an extremely interesting read that you are sure to enjoy.
From the Article:
Maybe the greatest mystery of the many mysteries surrounding the Mystic Lamb, was caused by the lower left panel, called the Just Judges. The original panel got stolen in 1934, has never been found and was replaced by a copy. It's Belgium's most famous unsolved mystery. Countless amateur and professional sleuths are still tracking clues. In a BBC interview with crime writer Minette Walters, Ghent's former police chief Karel Mortier referred to the theft as "the art crime of the century".