Was all set to post this in SSID but happily this subforum jumped up and hit me in the face before I could get there.
Met this one in today's 404s:
184.108.40.206 - - [04/Mar/2013:05:23:01 -0800] "GET /trafficbasedsspsitemap.xml HTTP/1.1" 404 930 "-" "Mozilla/5.0 (compatible; bingbot/2.0; +http://www.bing.com/bingbot.htm) SitemapProbe"
Some cursory googling hahaha
leads to this page
Apparently I got lucky with just the one request. Some folks have been getting so many that they've resorted to locking out bing. (Seems a bit pointless to me: unless you've got a firewall setup, a 404 probably uses fewer resources than a 403.)
Why they expect to find this server-plugin-generated file on a shared-hosting site-- especially one whose robots.txt includes an explicit sitemap reference-- must remain a mystery.