If these computers don't have installed "Symantec Management Agent" and only inventoried via stand-alone inventory package, then such computers will be shown as not managed by your Notification Server and they will have '{00000000-0000-0000-0000-000000000000}' in GUID column of Inv_AeX_AC_Identification sql table.
Simple query to create 'filter' which will shows these computers
SELECT _ResourceGuid FROM Inv_AeX_AC_Identification WHERE GUID='{00000000-0000-0000-0000-000000000000}'