itemsearch.pl Item search and searching 'collection is not' does not take into account items that do not have a collection code. Not all items have a collection code.
This seems to happen with some other values that can be NULL. In the drop down menu with the barcode placeholder there can be several different options working the same way, for example age restriction or series. If you search items while selecting these as "is not", the results will not include the items that have a NULL value. It seems to be the way SQL works. Even reports will not give you the NULL results if you search for items with (CCODE != collection) but you can circumvent it using (CCODE IS NULL OR CCODE !=CCODE). This is how our librarians think it should work in item search. (And if we changed this logic it would still be possible to do the CCODE search the way it was, by selecting 'Collection IS' and making the multiple choice of all the collections but the ones excluded.)