It sounds like the only thing requesting the 404`ed page was google bot, which I do not believe tells you the referrer. If this is true, then it would mean either that google does not clear their cache (which I doubt), or that the link exists somewhere on the net, but in a place where no human would find it. I've done some work with web crawlers, and it you fall into that type of hole alot more often than I would expect.