Dear friend,
It is possible she still smells where the other cat went because only an enzyme product like natures miracle or odor mute can completely remove the scent.However, if she has never shared your home with another cat, it is unlikely she is marking any territory. Most of the time when cats soil out of box they are trying to "communicate" illness or discomfort in the hopes of getting help.Some cats defecate inappropriately because they are concerned they will step in stool if they do it in a box. That most often ocurrs when the stool is loose, though. The fact that she urinates in her litter tells me she perceives an issue with the stool only. If she ever defecated in a box and was startled ar scared that could account for her behavior as well.
There are a few things you can try besides the enzyme cleaner since you have blocked her access to these areas and I assume she has found a new spot to soil. Give her a teaspoon every day of plain yogurt to keep "good" bacteria in her colon. Also a a daily drop of unsweetened (ONLY) cranberry juice to lower the PH of her stool and urine. Try "cat attract litter" which has been formulated for problem cats. Also feliway plug ins and spray exert a calming influence on cats. Beyond these suggestions, the only other possibility is to temporarily confine her to a room with no carpet and two boxes and see if she will use either for poop.
She sounds like a lovely cat except for this issue and you are very kind and patient to put up with such a thing. I sincerely hope that she will respond to some of these suggestions. Please keep in touch and let me know how she is doing. If you have further questions, please do not hesiate to ask. I will try to. help in any way possible.
Sincerest best wishes,
Terri
Dedicated to Cats and the people who love them!
Please press ACCEPT if my answer was helpful and leave feedback Thank you! Terri