Physical reasons: Not only UTI but crystal/stones are a possible cause of missing the box. Plus any illness that makes them feel "sick" when they need to go. Diabetics and kidney patients often miss the box because they can't hold it.

Behavioral reasons: If the cause is stress then feliway plug-ins might be what you are thinking of. There are also medicines to reduce stress.

BUT, if it is not stress it could be habit or defiance.

THINK: Has anything changed? Besides the obvious, food, litter, placement of boxes, has your routine changed? Any new members to the family? Stray cats bothering her through a window? Are there other pets? Do they possibly scare her away from the box? How often do you soop? (she might not use the box more than once if it is dirty)

If nothing can be found physically, you may need to "retrain" her to the box by confining to a small area until she uses the box exclusively.

Good luck.