The system preference 'automatic_item_modification_by_age_configuration', used in misc/cronjobs/automatic_item_modification_by_age.pl, does not show in a system preference search. I think this is because it's missing from ./koha-tmpl/intranet-tmpl/prog/en/modules/admin/preferences/*.pref
It's a tool, not a system preference.
Should explain that a little more, the configuration is set in its own tool and then added to the systempreferences table for storage. Once you have set the first rule, it will appear on the 'local use' tab. This is a trick similar to the one used by the 'did you see' feature. It's intentional it's not showing up in search as you are not supposed to edit the rules by hand.