A ferret!! You know, when I read that it seemed really familiar. Somewhere I had read/heard that a ferret's odor is quite offensive to a cat
I am sure that the odor will linger for A LONG TIME (even if you can't smell it, your cat can ). I would find out what to do to get the ferret-smell out of the carpet/floor/curtians, etc. If it's already been quite awhile, I don't think it will go away on it's own.