Note: This is probably not a general answer!
Joachim was right. It turned out that the googlebot is not guessing URLs.
Doing a bit of research I found out that I added a new DIV in my site containing those special URLs half a year ago (which I unfortunately forgot). A week ago googlebot has started crawling it.
My solution: I deleted the DIV and also I return a 404 status code on those URLs. I think, sooner or later, googlebot will now stop crawling the URLs after revisiting my site.
Thanks for the help!