> I don't recall many sites that would have good information for a search engine in that string.

Well, Google is one such site - as in, if you have bing toolbar installed and are on google.com/search?q=keyword and click on example.com, then Bing can easily extract "google com search q keyword" and associate it with example.com - without anything explicitly or intentionally relating to Google in their code.

They may also be looking at referrer info.