A biblio entry without any items attached to it will show up in searches and as an advanced search category when set to hidden using the OpacHiddenItem field. Example: You have an item type with a code of FOO. You set OpacHiddenItem to "itype: [FOO]" You have a single biblio entry of Bar. Bar does not have an item in it. Expected result: Bar is hidden from searches, and is not a category in advanced search Actual result: Bar is visible in searches, and is a category in advanced searches Workaround: Add an item to Bar
See MARC code 942$n for OPAC Suppression of the Biblio record without an item.