Sam -
 
I'm so sorry to hear of your loss and I hope you are able to find an
answer soon.
 
Unfortunately, with no noticeable symptoms there are too many
possibilities to hazard a good guess.  I would suggest you arrange for an
autopsy, especially if your ferret was younger, to see what happenned.
 
Deepest condolences,
Lorraine
 & Al the sock warrior
[Posted in FML issue 4197]