Recently I did an article on Where are autoresponders and forwarders files on a cPanel server but what about email filters?
Filter files for individual email accounts do not exist by default, but are created when user adds filters or cpanel user disbales delivery for that account.
They are created in etc/$domain/$address/filter
so for stefan@example.net the file is etc/example.com/stefan/filter
To list all accounts that have (or had) filters simply search for files named filter
