appearance_rates() returns a $monster[none] record for The Haunted Library
Mafia build 19477.
After banishing the librarian and the desk (which doesn't appear to have anything to do with it, but it's just where I was when I noticed this), running
> ash appearance_rates($location[the haunted library]);
Returned: aggregate float [monster]
none => 25.0
bookbat => 75.0
banshee librarian => -3.0
writing desk => -3.0
AFAIK, none is not a valid monster to return. What I presume is the same bug also exhibits on the Location Details tab of mafia. See attached screenshot.
This is how Mafia denotes noncombat rate.
(You'll note that the location details tab in your screenshot also indicates that Combat Rate is 75%.)
Wow, good to know. Sorry for the spurious report!