When I had Star, she used to go outside so we abandoned the litter tray. She started doing this in obvious places because she had developed a urinary infection and was no longer "lasting" as long, even though we had a cat flap. Vet fixed that.

She did it again a couple of years later when Squitten the kitten got run over, I think that was stress and distress at being the only cat again. She needed fuss and reassurance.