Some cursory googling hahaha leads to this page [bing.com].
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.