How to tell if it's retaliation

[karate0kat]

Well, I'm in a studio apartment, so sadly there is no door to close, but I could definitely try laying something over it when I'm not using it.

[ibbica]

Originally Posted by karate0kat

She did it again!
...
I have no idea why she keeps peeing on that specific sheet set!

Probably because she can still smell her pee on it, and so now thinks it's an appropriate place to go I'd try cleaning it more thoroughly; you can use an enzymatic cleaner that will break down the urine components, vinegar, or hydrogen peroxide (tends to bleach colours, so test on a small area first if you're concerned), or some combination of those. A quick 'net search will offer all sorts of more specific recipes