On August 21, 1911, the Mona Lisa was stolen from the Louvre. The ensuing media frenzy was unparalleled for any sort of art theft, and the hysteria bred all sorts of conspiracy theories. One prominent theory was that it was stolen by rogue modernist artists, trying to destroy the old world order and imposing their own. This line of thinking actually led to the arrest of Pable Picasso as a suspect, but he was never convicted. 

The painting was actually stolen by Vincenzo Perugia, who had briefly worked at the Louvre. Wearing a worker's smock, he hid in the museum until after it had closed, took the painting out of the frame, and hid it in a closet. The next morning, he hid it under his smock, and walked right out with it. He kept it in a hidden compartment of a trunk in his house for the next two years. 

Britannica, The Editors of Encyclopaedia. “Mona Lisa.” Encyclopædia Britannica, Encyclopædia Britannica, Inc., 4 Apr. 2019, www.britannica.com/topic/Mona-Lisa-painting.