I posted this in the heating/air conditioning forum but will try my luck here as well.
We've lived in our house for 6 years and I finished the basement 2 years ago this December, 1300 square feet, about 300 of that is a storage room where we keep a litter box for 1 cat. We change the little once or twice a week and the cat is very clean. I never noticed the smell (before or after the basement was finished) until about a month ago but now when I get to the bottom of the basement stairs, I can smell the litter box (the storage room door is way across the room). I know there's no accidents on the carpet anywhere either, have looked extensively into this.
I go into the storage room and it isn't strong at all. Here's my hunch: The smell started when we turned the air conditioner off for the year. I turned it back on and the smell is gone. Turn it off, smell reappears. Does it have to do w/ circulation? One other thing, I haven't put in the return airs in the basement yet. Could that be it? Any advice is much appreciated.