bhartzer - 6:11 pm on Dec 28, 2010 (gmt 0)
This happens all the time. Most likely someone has made a link to the URL that includes the tracking code, and the search engine spider has crawled the URL.
If you run PPC ads and include tracking parameters, people will click on the links (which they're supposed to do). They will end up bookmarking the URL and sometimes they actually will make a link using that URL.