You probably have something that links to these resources.
(perhaps some URL you are unaware of or some script that generates links by mistake)
You can find out more by setting up a Google Webmaster Tools account. After a day or two you should start seeing some crawling errors, for each you also will be able to see the page the links to the nonexistent urls.
If you don't get any crawl errors, then you should see if it's really google that visits you or some kind of impersonator. (Spam bots and SEO monitoring tools love using Google user-agents to bypass security and/or get a "Google-view" of the site)
For this you can use Bototpedia Googlebot IP verification
Just put the visiting IP in the search box and it will tell you if its valid for Googlebot, or not.