Cleo has not been using her box.

We thought we smelled urine in the basement in the past but could never find
it. About six weeks ago we finally discovered that she had been using a sofa
in the basement for her litterbox.  We cleaned it with Nature's Miracle and
washed the seat covers, but it still smells. Covering the sofas in plastic
seemed to cure her for a while, but then she started pooping beside the
litter boxes. Although not a good thing, it's much better than whizzing on
the sofa.

About two weeks ago I thought I'd smelled urine again, but wasn't sure if it
was the sofas outgassing or a new problem. I was giving the fish their
weekly bath (water change), when I noticed Cleo going into the poop room. I
saw her checking out the one litterbox, so I thought all was well. A few
minutes later I caught her peeing on the carpet in a corner. I know where
her new spot is now!

I've read about the various reasons cats do this (texture, smell, poor
training as a kitten, UTIs, marking, etc) and some of the deterrents and
training methods. I'm pretty sure Cleo does it because of the smell of the
other cats (VERY territorial), although it's possible she associates it with
the pain from her Spay. I'm not sure deterrents for the current spot will
buy us much, as there are just too many other choices. I also did not think
much of the advise to lock her in a room with a box, until she learns to use
the box again. This seems somewhat cruel and likely ineffective, if the
other cats are the issue. She did not seem to have any problems using the
box when we had her segregated pending her medical. But of course she was
the only one using the box then.

Any bright ideas? I was considering getting Cleo her own box, using one of
those locking cat doors that use a magnetic collar. But I don't know if this
is the real issue. I guess I'll segregate her for a few days with her own
(clean) box to see how she makes out.