So far so good.

We have two diffusers on the first floor. Louis has sprayed (or
squatted and peed) in four locations that we're aware of, three
downstairs, one upstairs.

He has stopped peeing upstairs for reasons unknown (actually, he only
did it twice). .

Downstairs, we have a barricade in front of one spot, have blocked off
another spot (it's a bathroom and the door is closed) and a litterbox
at the other spot with "Cat-Attract" litter. We've had no
inappropriate behavior in weeks.

I did verify that there's a new neighborhood outdoor-cat. But this
doesn't seem to have been the issue.

One interesting thing is that we've noticed that Louis is pretty
nervous about the basement (where the litterboxes have traditionally
been). I've set up an office down here to get some peace and quiet
while my son's practicing his trombone or my wife's practicing on the
piano. Louis will stand at the top of the stairs and perform little
voice recitals for a long time - but he won't come all the way
downstairs, even when I call him.

Tonight I brought him into my office, and put him on my desk (he
usually enjoys hanging out wherever I'm tapping on my laptop). He was
in full "OMG" mode - crouched.. ears back, shrinking from my patting.
He wouldn't even jump off the desk. So I finally picked him up and put
him back on the stairs, where he's currently sitting, serenading me.

So, the current theory is holding out - that something about the
basement has freaked him out (probably Henry the stray's recent
captivity), and unable or unwilling to use the litterboxes down here,
he's found spots upstairs to go.

My wife wishes we could get rid of the upstairs litterbox - but the
current plan is to slowly inch it inside the closet it stands in front
of. If we can see Louis happily frolicking downstairs (which my
repeated presence down here may encourage), we may slowly inch it
toward the basement and see if he starts using the litterbox down here
again. In the meantime, we've started opening spot #2 (the bathroom)
up for a little bit each day and keeping an eye on him. We've
seriously scrubbed it and doused it with "Anti-Ickypoo" (which
actually is a pretty incredible product) and he sniffs, but wanders
off disinterested.

I'll keep y'all updated. I hate to think of peeps who euthanize their
cats because they pee in the wrong spots. So far this has been a
fairly straightforward troubleshooting session.

BLink