Here's one more.  As far as I can tell no one else has mentioned this:
food.
 
I fostered a pair of ferrets who really stunk when they came to me.  I
cleaned the corners of their cage and all their bedding - threw some out -
and it helped - but there was still a noticeable stench.
 
I removed their food dishes - i realized that they stunk too.  They were
made of metal, & I couldnt clean the smell away!  Throwing them out helped
too.
 
The smell on the dishes came from their colorful, but fish-based, food.
After a day or 2 I started mixing their food with TF, to see if i could
get them on a better diet.  They were willing to make the switch, and when
they were finally off the fish based food - the smell all but vanished.
 
I think when their diet has a lot of fish in it, the smell gets in their
fur, their litter boxes, everything absorbent in their cages - and even
some things that shouldnt be absorbent.  We might not recognize the smell
as being fish - but it can be strong & unpleasant.
 
Even some of the more nutritious foods are based on fish - check it out.
[Posted in FML issue 3252]