I think your cat is developing new habits, even if you haven't found the accidents yet. When you say you haven't put in returns yet, do you mean no grills? Maybe it climbed in and went exploring, etc.

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.

It's definitely not the cat going other places. I even got the black light out and got on my hands and knees and went all through the basement smelling the carpet last night and there's nothing. Carpet actually still smells new.

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?

Have you actually seen a blacklight work effectively that way? (I tried it for rodents and the light was ineffective.)

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?

The black light is very effective for cat urine, and I crawled and smelled every inch of that carpet and there's nothing. And it doesn't actually smell like cat urine, it smells like the litter box (if that makes any sense).

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

Dehumidifer. The AC was providing dehumidification, and the dryer air suppressed the odors.

A dehumidifier did the trick, thanks for all of the advice.