Yeah, that's a classic caper. The interesting part here is that the thieves never intended to keep the painting. They just wanted to be able to make remarkably good, hard to detect forgeries and -- while the painting was thus known to be mssing -- to sell these forgeries to unscrupulous millionaires who would pay a fortune to own the Mona Lisa even though they couldn't display it without being in serious trouble. They did okay. Once they had their money, they returned the real Mona Lisa to the Louvre.
The venality of some people is simply unbelieveable. There are people today would pay millins of dollars for a painting they can't even admit they own, let alone display. These rae not art lovers. They call themselves investors.