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.

Also, I would get a friend who doesn't live with animals in the house and use their nose to trace it. They'll be much more sensitive.

I haven't even installed the return airs yet. I was thinking that might be the reason since there's probably no air circulating down there?

How about letting someone catsit and remove the litterbox for a few days and see if the smell remains? (I'd imagine that it will linger.)

And no offense, but I've seem too many "clean cat" situations that deteriorated at some point. The owners are the last to know. Perhaps that's not the case here, but that's been my observation. (BTW, male or female? I've heard that male cats can really get into marking territory - do you know if that's true?

I think I'm going to start w/ a dehumidifier, that may help the problem.