Dingbat mcgee me decided that it'd look better to have the flooring straight, rather than cockeyed like the vent (darn person whomever put it in....). But, that meant I locked in the vent when the flooring went in.
The smell got worse.
"We really need to tear the floor up to see what it is, Jess....." said my husband.
Finally, I capitulated, and he went all gun ho happy on the floor with the crowbar. He was a little TOO happy about it, if you ask me.
Long story short, we discover that vent actually isn't a true heat pump system vent... it's connected to the crawl space so air can flow into the closet from the top vent (a true vent) and then out into the crawl space.
We deeply suspect that a squirrel or some other vermin died in the crawl space. We're debating whether we should try to find it, or if we should just let nature take its course and let the smell go away that way.