Meaning?

Hi, what is the meaning of thoroughly unexpected sight of Bob in the following text:

The wall behind Bob was plain white drywall, or so it seemed. A section of it was a hidden screen, and behind the screen were three cameras aimed at Karen Sharbonnet 's face.Every twitch and blink was being analysed by their experts. Every movement of her eyes and the muscles across her forehead and around her mouth. She was ice. Her hands were frozen. Her breathing calm. Her expression never changed. At the thoroughly unexpected sight of Bob, she had registered .

From the book Camino Winds by John Grisham.

Thanks.