I'm not too surprised. I've got Googlebot still requesting old URLs even through there are no incoming links to them (that I know of) and they've been either 404 or 301 redirected for six months. I even tried using 410 Gone instead of 404, but it made no difference.
To just reiterate this further, I am still 301'ing urls that have been dead for nearly 5 years. I still get requests in for them. I don't want to 404 them in fear of losing that slight bit of traffic so I just 301 them. I am really surprised they don't remove these urls from their cache and I can't think for the life of me why they don't?
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.
I removed a whole section from the site at the same time. Webmaster Tools shows the incoming links for every page in the section as other pages in that section. It's a whole loop of pages linking to each other and generating inbound links even though they all no longer exist and haven't for many months.