[exim] orphan bounce

Hi,

Please consider this line from the Exim (4.72) mainlog:

2012-01-08 05:17:20 1RjkC8-0007uf-24 <= <> R=1RjkC8-0007ue-15 U=Debian-exim P=local S=1732

As I understand it, message 1RjkC8-0007ue-15 provoked a bounce, which is
fine, but there's no other trace of 1RjkC8-0007ue-15 in the mainlog.
How is this possible? If I send invalid mail via exim4 -i ..., that
doesn't appear in the mainlog either, but the R field isn't present in
the generated bounce:

2012-01-11 18:09:23 1Rl1fi-0006uz-9R <= <> U=Debian-exim P=local S=759

What mechanism generated the above log line then?
--
Thanks,
Feri.