Bing didn't get there by guessing. Somewhere there is a link to either the folder or one of the files inside it.
Bing found various files *inside* the folder (which are definitely not mentioned anywhere) and even passed valid CGI parameters to them
It definitely didn't achieve that by guessing. Somewhere in the universe there exists a reference to the pages. If it were, ahem, a different search engine, I'd look more closely at gmail. Bing isn't furtively trying the same thing with hotmail are they? Or whatever mail it is they own this week :( Matter of fact it would almost have to be something that's only visible in Redmond, because otherwise the googlebot would be all over the pages too.
I don't suppose they're doing anything fantastically useful like listing the pages in bing wmt with details on Who Links Here. Nah, too easy.