Jonesy - 4:57 pm on Sep 29, 2012 (gmt 0)
I had such a problem some time back.
It was (eventually) easy to trace due to the fact that one
other fella (several states away) and I were working/testing
a PHP script in a "hidden", unguessable directory.
He had a (Alexa? -- pretty sure) toolbar installed in his browser.
His browsing activity leaked out that way.