But like my 'email canary': <a href="http://blog.jgc.org/2011/06/my-email-canary.html" rel="nofollow">http://blog.jgc.org/2011/06/my-email-canary.html</a>
There is a much better (and older) idea: introduce subtle watermarks into the actual content of the document itself. Minor typos, changes of word order, maybe even different facts. Different people in your organization get access to those slightly different versions. Once a document emerges where it's not supposed to be, your knowledge about who might have leaked it increases.<p>I have heard (not confirmed) that mapmakers introduce small errors into their maps to detect competition copying their maps instead of the terrain.
I've been doing this for a while with <a href="http://trackmycv.com/" rel="nofollow">http://trackmycv.com/</a> basically you can embed a transparent pixel in an MS word (or any other kind of MS office document) and get notifications whenever it's opened.
This is why I would never heavily use a computer without something like Little Snitch (<a href="http://www.obdev.at/products/littlesnitch/index.html" rel="nofollow">http://www.obdev.at/products/littlesnitch/index.html</a>) installed.<p>Highly recommend.