Langdon is obeying Neveu's orders, despite thinking she may be a bit touched in the head.
They meet in the men's room. Sophie tells him that he's Fache's primary suspect for Saunière's murder, and as such is under strict observation.
This is confirmed when Langdon finds the GPS tracking dot Collet had snuck into his pocket back in his hotel room.
Sophie tells him that the reason Fache is so sure that he killed Saunière is because there was a fourth line in his dying message that read: "P.S. Find Robert Langdon."