My vote goes towards a dead animal - mouse, squirrel, chipmunk, etc. My parents have squirrels that get into the space between main and second floors, then die. It stinks to high Heaven and sounds like what you're describing.

Do you have a crawl space?

Yep, it's a dead animal smell. One that you can't forget once you've smelled it.

We have had dead animals in our former house...very distinct smell. Now be ready to be grossed out....if you start having tons of flies in your house, that's where they're coming from. We had to have someone come in & spray in our attic space. He didn't find dead animal, but that's what it was. My DH also had one under the hood of his car! Took the car to the mechanic, and guess what he found!!

Totally agree with the above - I'm in the dead animal society!!! Probably a mouse - those little things make a BIG stink. I came back to college from Xmas break to a dead mouse behind the radiator in my dorm room - you never forget the smell. The good news is, once the offender is removed, it's not hard to get rid of the smell.

Yep -- DH and I vote for the "dead animal" -- probably a rat or a mouse ....

I'm voting for the dead animal too, and if you've been noticing for a week or so, it's a small one, will hopefully 'dry up' quickly. Anything larger and you probably would be camped out in a hotel.

Many years ago we had a 1920's house with a half basement, other half under the house was dirt/crawl space, and there were a couple of areas between the two under the house floor where something could squeeze in. Who knew.

The 'scent' was wafting right out of the shelved closet in our bedroom. DH fled to his fathers farm and left me to deal with the pest company, who had told me they were prepared to find it, or they had some kind of odor killing spray they could inject into the wall if that's where it was determined to be. He found it, by pulling down ceiling insulation in the unfinished basement. Rat, along with several of those little packets of poison that people can toss out for exterminating them. Never did find which neighbor was using the rat bait...

We sealed that area that could allow access, had a neighborhood meeting regarding kitchen compost, bird feeders, pet dishes, and everyone regularly cleaning up under any fruit trees - back then, there were a few neglected apple trees and 'ornamental' plum that were bearing fruit no one wanted and it was being left on the ground.

Dead animal smells nothing like a cat litter box.

Could it be the smell of a humid basement, or the smell of cement? You mention the "smell of a litter box", but is it the litter smell or the litter when it's been peed on? Clean litter can have a smell like cement, so I'm thinking that's what could be coming up thru the vent. We had a crawl space in one of our homes, and I remember it having that sort of smell.

It's not the smell that lady bugs give off, is it?
I'm trying to remember which corner of a house they like to winter over in -- southwest?

You reminded me of the time when we were at the old house and would occasionally get a whiff of something unpleasant, but so fleeting, you just weren't sure. Then it got stronger, and then DH smelled it too. We were going around the house on our hands and knees like a basset hound trying to identify where it was coming from. We finally isolated it to the wood pile. Yup. Cat had brought in a mouse that was in bad shape, but alive enough to get away and hide at the bottom of the wood pile where it died.

But I agree that dead animal is a very distinctive odor and would not be mistaken for cat pee which has a fragrance all its own. But dead animal seems likely if it's coming from a vent....

Rats give off the smell of "cat pee" or the equivalent as that is what they are doing under your house or in your duct work. They chew their way in and make nests...we have had an ongoing battle with roof rats for a couple years. They don't stay in the roof..sigh...they love our crawlspace. We had the fly problem in one bathroom also and it was indeed due to the death of at least one of the rats and the follow up of maggots/flies. We are very fortunate that our exterminator is relentless and energetic and agile :) He gets under the house and finds that corpses and removes them and places a deodorant and we go on for months till it happens again.

The biggest worry is that your duct has been compromised. Of course it has to be repaired as the condensation problem will be terrible when you use your a/c with a hole in the duct as well as your cold or hot air leaking out. I hope that it is not part of the problem . c

I have heard people complain about a dead animal smell, or fishy smell, and it turned out to be an outlet that had gone bad. I notice you have an outlet by that vent. You may want to check that out.

Well, the smell has mysteriously gone away. After a week. Weird. But thanks for all the ideas!

YEAH !!