It's probably the third page "fragment" of the "translated" page produced by Google Wireless Transcoder. This is a proxy that re-formats your pages to fit on small screens, translates them to XML+XHTML or WML as required, and sometimes breaks them into several smaller pages for viewing on mobile devices.
To see it in action, go to www.google.com using a cell phone or PDA, do a search for your keywords, and then click on one of your longer non-mobile (HTML) pages in the search results. GWT will reformat your page and translate it into the markup language your device uses, and you'll see the requests in your logs with the "Google Wireless Transcoder" user-agent.
If you are not seeing the GWT user-agent in your logs, then perhaps someone has copied the GWT URL from the search results and published it somewhere, and Google is now following that link.
Thanks, It jogged my memory after I posted, and I actualy asked a similar question here about 18 months ago. It appears to be comong from when someone uses the table of contents after landing on a page from google mobile [google.com...]