One of the cats is pooping downstairs. I only have two cats, so it has to be one of them. As I found two piles in the same corner, my first hope was moving one of the cat boxes to the same location would solve this rapidly escalating situation quickly and quietly. I was mistaken. The next day there was a pile on each side of the newly placed cat box.
Erroneously thinking I was smarter than my cats, I understood what had to be done next. I swapped who's box was the downstairs box - I had obviously mistaken which cat required his box down there. I felt good about swapping them when I discovered the only cat who goes into the garage was also surreptitiously pooping there as well, which obviously confused me since I was cleaning poop from both cat boxes.
But that didn't solve the problem.
Further investigation is required.