This is going to sound a bit strange, but this is my best guess on why a
ferret who has lost a mate would stop using the litter box:
 
It is a well known fact that dogs use their urine as a marking.  Many
animals do this.  I am thinking that quite possibly a ferret who has lost
it's mate would leave markings in a wide area to guide the missing ferret
home.  I think Misty might be doing this.  I am guessing that Misty had
no idea what happened to her mate, and this is her way of looking.
 
This is my own hypothesis, so take it for what it's worth, but this would
be my best guess as to the behavior.
 
Roary
Albuquerque, NM
blog - http://ferretphilosophy.blogspot.com/
[Posted in FML issue 5039]