I found something inside the sieve_filter.master and sieve_filter_1.2.master that looks like the subject line. I do have a mailserver on a different machine, not virtual.
Multi Server Setup, with four server!
So I have the ispconfig /conf /custom-conf on both machines and probably on the DNS1 and DNS2 machine, but I am not concerned about them.
I suppose that the one or two that need modification are on the mail server.
In any case I went and modified whatever I could find.
An interesting observation I made, was that if I get an auto response from the master server domain, (ALSO THE MAIL DOMAIN) it's the "Out of Office" subject line. If I get an auto response from a different domain, I get my modified version.
I thought I had them all, apparrently I must have overlooked something.
Thanks for your input!