1script - 5:05 pm on Sep 27, 2011 (gmt 0)
See if there is any hits on the page "this_page_should_not_exist.fake", then you have the IP. Seems like they sometimes do check for a proper 404 response using this filename.
Not this exact URL but Googlebot does probe every couple of days for
/noexist_googlesiteid_.htm where '_googlesiteid_" is the same as in my WMT verification URL /google_googlesiteid_.html
Come to think of it: I'm looking at this 404 page on my site and it meta-refresh redirects with 1 second delay to the homepage. Dumb move? Could this be looked at as a 301 of any junk pages including spam and DMCA-ed to the site's homepage?