Search engines tries to access node editing links
One possibility is that one or more logged-in users have Google Toolbar or something similar installed. I don't have it installed any more because I noticed when developing locally that every time I fetched a page in my browser the request was repeated by the toolbar, presumably sending it to Google. 

In my experience there is no such thing as a non-public URL as far as Google is concerned. Any URL can be listed for indexing and so crawlers may attempt to fetch it from time to time. I suspect other tools may record URLs in a similar manner.