Throughout the novel, Dan Brown keeps making references to a knight in shining armor that sits upon Saunière's desk in his office at the Louvre:
One of Jacques Saunière's favorite pastimes was bringing Da Vinci's more obscure brainstorms to life— timepieces, water pumps, cryptexes, and even a fully articulated model of a medieval French knight, which now stood proudly on the desk in his office. Designed by Da Vinci in 1495 as an outgrowth of his earliest anatomy and kinesiology studies, the internal mechanism of the robot knight possessed accurate joints and tendons, and was designed to sit up, wave its arms, and move its head via a flexible neck while opening and closing an anatomically correct jaw. This armor-clad knight, Sophie had always believed, was the most beautiful object her grandfather had ever built … (47.21)
At first, it seems like a little bit too much information about an object that was solely described in order to establish Saunière's obscure hobby. Collet mentions being a bit creeped out by the knight as he sits at Saunière's desk. Fache stares at it while monitoring Langdon's tracker.
So why does Brown keep mentioning this bizarre artifact?
Well, Brown emphasizes the knight because it becomes an important bit of imagery that he wants us to keep in mind at the end of the book…when it turns out to be a major plot device. Had he not made it clear that this item was a prominent figure in Saunière's office, this would've seemed a bit too convenient:
He was holding a photocopy of an ancient schematic diagram, which depicted a rudimentary machine. He was unable to read the handwritten Italian labels, and yet he knew what he was looking at. A model for a fully articulated medieval French knight.
The knight sitting on Saunière's desk!
Collet's eyes moved to the margins, where someone had scribbled notes on the photocopy in red felt-tipped marker. The notes were in French and appeared to be ideas outlining how best to insert a listening device into the knight. (90.16)
Ah ha. The creepy knight doll is in fact a Trojan Horse, planted by the devious Teacher in order to hide one of his surveillance bugs. How clever.
The moral of this story? Don't accept candy from strangers…and don't accept weird desk tchotchkes from creepy friends.