If you believe, as Maurizio Seracini does, that Leonardo da Vinci’s greatest painting is hidden inside a wall in Florence’s city hall, then there are two essential techniques for finding it. As usual, Leonardo anticipated both of them. First, concentrate on scientific gadgetry. After spotting what seemed to be a clue to Leonardo’s painting left by another 16th-century artist, Dr. Seracini led an international team of scientists in mapping every millimeter of the wall and surrounding room with lasers, radar, ultraviolet light and infrared cameras. Once they identified the likely hiding place, they developed devices to detect the painting by...