Possibly a Google anomaly or can someone actually hide their search terms from Google?
The usual portion of the log entry, i.e., "http://www.google.com/search?q=my+search+term..." was instead empty quotes: "-"
However, I definitely got charged for the click and can deduce the general search phrase used by examining my campaign stats.
I don't know enough about this stuff to have an informed opinion, but maybe it's a network security thing on the users part?