I have an alert service where users can create custom news alerts. In this process unique URLs (which contain a hash value) for verification and deletion are created.
Now I'm seeing one of these URLs "id=...&hash=" in Google's index but not in bing's or Yahoo's index. I'm not not Adsense or any other kind of advertising on the page. I'm not using Google analytics.
I'm just seeing two possibilities how Google has found that URL: either is was due to the bowser/toolbar or identifying them in the e-mail (of a Gmail account). Unfortunately the entry in the database which belongs to that id is already deleted. Therefore, I doesn't know if it was a Gmail account or not.