After deleting a patron if you go to that patron's record via: https://staffsite/cgi-bin/koha/circ/circulation.pl?borrowernumber={{BorrowerNumber}} You get a page that says Checking out to () and no data (which is expected, since the patron doesn't exist anymore in the borrowers table.). If you click the details page, you do get the error message: "This patron does not exist". Would it make sense to have this page also say "This patron does not exist"?
Fixed in Bug 16082.