I had a similar problem with a cat once. She kept peeing on my bathroom mat or towels on the floor. I came to the conclusion that it was something in the smell of a damp towel/mat that made her think it was OK to use as a loo.
i cured the problem by clearing the floor of all towels and mats so it wasn't tempting for her, and temporarily putting a tray in the place where she was going. Another option is to put a bowl of her food there...cats never pee on their food.
In my situation, I think the cat was a bit stressed at the time & it's worth giving some extra fuss to your cat & observing if there might be something causing her stress. Also, if it continues, you'd better take her to the Vet & check that she doesn't have cystitis & that can make a cat who is usually clean to urinate out of the litter tray because it associates the tray with pain.