Factual error: When they first pick up the Mona Lisa, they show the back. There you can see a canvas sheet over a wooden framework. However, the Mona Lisa is painted directly onto wood, no canvas at all. The scan they run even says it's painted onto wood, despite visual evidence to the contrary. (00:06:35)
Factual error: The real Mona Lisa is much smaller than the 'authenticated' painting shown in the film. (00:06:30)
Answer: Since Preston is the one who discovered Partridge's sense-offense and executed him, he was probably required to give a report of how it went down and mentioned Partridge's last words, which one would expect would make its way to DuPont.
Phaneron