I read Da Vinci Code a while back (http://www.livejournal.com/users/marionravenwood/42819.html), and while my reaction was not totally unlike yours (I felt like I was reading an action movie),the mysterious hand (http://easyweb.easynet.co.uk/giorgio.vasari/vinci/vinci19.jpg) at least is for real. It's the hand holding the knife. If you looked at a restored version, it might not have been there.

What the hand actually means, of course, is still open to interpretation. I wish they had actually put plates of the art in the book; I kept having to get out my old art history textbook.