[exim] How to create a filter to catch all bounce messages

As a follow-up to my previous question. How does one go about
creating a filter that would match all bounce messages?

I have tried:

if error_message then
    seen save /nm_var/bounces/
endif

But that doesn't catch the bounces that exim sends. For example if I
do:

-----
exim singletoned@???
To: asdfasdfasdfasdfasdfasdfewrqertrtyrthsfgdfgadfqeadvxzvz@???
Subject: A bounce test

Another bounce
-----

Then that gets bounced to my gmail address.

The filter that I tried that caught all my mail was:

if $header_return-path: is "" then
    seen save /Users/singletoned/temp/exim/var/
endif

As before, any clues or ideas will be gratefully received.

Thanks

Ed