The Collector

For only the fourth time in two centuries, the French have allowed the Mona Lisa to leave the Louvre, this time at the request of the pope, who wants to exhibit it in the Vatican Museums. However, once on display, the Museums' former curator notices a nearly imperceptible discrepancy in the painting, leading to the discovery that it's a forgery. Faced with the crisis of losing the most valuable painting in the world, the pope turns to Mauro Bruno and his associates, who'd previously performed discrete investigations for the Vatican.
As they begin, Bruno and his colleagues try to understand how the world's most famous painting, which is kept inside an alarmed environmental enclosure fifteen feet from the nearest person and under constant visual and physical surveillance, could be stolen. Even in transit to the Vatican, it was under continual visual monitoring with a security team inches from the masterpiece's enclosure, leaving the investigators bewildered about how a theft could have occurred. However, they soon learn that the Mona Lisa is one of many heavily guarded museum masterpieces that have been stolen and replaced with a forgery, the thefts having gone unreported.
With only two weeks before the Mona Lisa returns to France, and not having a clue as to the identity of the mysterious person they call the Collector, they must find and retrieve the Mona Lisa.