Langdon's totally shocked that Saunière wrote his name in his dying postscript.
He's also pretty miffed. Why would Saunière write his name? Especially because it would make it look like he was the murderer?
Sophie explains that in France, the laws protect the police, not the criminals. Fache is justified in arresting Langdon, who is basically up a creek without a paddle—he looks real guilty.
Sophie confesses that she knows Langdon's innocent, and it's probably her fault he's in this mess.
Turns out, Saunière was Sophie's grandfather. Even though they had a falling-out ten years ago, she knows this message was for her (Vitruvian Man was her favorite Da Vinci work, and P.S. stands for Princesse Sophie, her childhood nickname. She thinks he included the Fibonacci sequence to guarantee her involvement as a cryptographer.)