Have vet check but that location sounds like it's the end of spleen.
Enlarged spleens are not unusual in ferrets and many seem to be related
to low grade chronic infections such as helicobacter or a history of a
large acute one such as ECE.  If one gets large enough to pose a risk of
hemorrhage removal is easy and they don't miss them afterwards.  Even not
enlarged, the end of the spleen can be felt below the rib cage on the
ferret's left ventral side.  It is also possible at times to feel large
fecal masses when they pass through from the transverse to the descending
colon in that general area.  If you combine an enlarged spleen and a ferret
prone to large fecal masses or to constipation you can have a situation in
which the two bump up against each other and massage can help then till
correction is made -- the vet can show you.  Have a vet feel the ferret
along with you so that you can learn what is normal and what isn't for
various locations: spleen, nodes, abdominal inflammation, etc.  This is
a good thing for anyone to do.
[Posted in FML issue 3509]