Hi, I wanted to update everyone about this problem, which I first posted in February.
The bad smell has been located.

A few days ago, a couple of construction friends of my homeowner friend came over
and removed a section of sheetrock from the corner of the dining room closest to
the garage. Right away, they spotted a long line of beetles moving inside the wall.
They followed the beetles upward to see where they were feeding. They found a large,
black, dead cat lodged in the wall. It looks like the cat entered through the garage
and entered the wall (it was wintertime in Minnesota) between the ceiling of the
dining room and the floor of the bedroom above....and he got trapped. This explains
why the smell didn't go away, even after five months.

I told my friend just a few weeks ago to borrow the neighbor's hunting dog to sniff
arouond, but he didn't take action. He's at work all day, and his wife stays home,
so it wasn't so urgent for him. Anyway, a couple of construction worker friends from
church convinced him to open up the sheetrock, and the mystery was solved.

I'm really glad they won't have to put up with that lousy smell any more. Sometimes,
you just have to get in there and tear things apart.

Thanks for all your suggestions!
Scott