Hey everybody, thanks for the comments. All very helpful as always.
I have a suspicion it is Penny, the 16 yo, who is pooping outside the box (there is never urine). It has occurred in the vicinity of all three boxes. I've been adding Cat Attract and cleaning with Nature's Miracle.
Penny does also vocalize frequently, sometimes for attention, sometimes for food, sometimes I wonder if she's in pain, but it often seems like for no reason whatever. So the idea of dementia is in the forefront of my thoughts with her.
Penny still jumps to the top of a tall king bed, so I don't think arthritis is an issue, but I definitely have seen benefits of low-entry boxes at the Shelter, so we might get one of those. Although, Nala seems to be a cat who often hits the side of the box instead of into the litter at the bottom, so it can't be *too* low.
There are no outside animals that I've seen and no previous residents, as we built this home. So I don't think an external or previous pet is to blame.
The furnace/water heater aren't noisy. I don't think noise is a factor.
With regard to CKD, I am fairly familiar with Tanya's site, although I haven't joined their community. We will continue to get regular bloodwork, and when the time comes for SubQ fluids, we will do that. However, I will refuse to make their lives more miserable with frequent vet trips, blood sticks, fluid sticks and pills - there will be a time (and I probably won't recognize it) when too much is too much.
Oh and yes, Patty, the wedding was June 9
Thanks for all the input again. You guys are awesome.