Langdon and Sophie start searching the area around the Mona Lisa for any clues with their blacklight.
As they do, Langdon fondly recalls his lecture to a bunch of inmates about the infamous painting.
His theory's that the Mona Lisa is a huge practical joke by Da Vinci, because he did little tricks like make her look larger from the left than the right, insinuating that her femininity was superior to her masculine qualities.
And "Mona" is an anagram for Amon, the Egyptian god of fertility (and known for his raging masculinity), while "Lisa" is a nod to his feminine counterpart, the Egyptian goddess of fertility, Isis, who was also known as "L'ISA."
(To make a long story short: Mona Lisa is totally androgynous, and her name is an anagram of the divine union of male and female. So his big joke is that Mona Lisa is not a woman.)
Sophie spots a drop of blood on the floor, so they know Saunière was there.
Finally they think to shine the light directly on the Plexiglas in front of Mona Lisa's face, and see six words glowing in purple…
But we'll have to wait for a new chapter to find out what they are.