Why oh why has our cat stopped weeing in her litter tray?

[flapinko]

She is a very nervy indoor rescue cat who we got at 2 and a half years old.
She has always used her litter tray no problem, and we keep it clean, and change it 1-2 times a week.

However, we noticed that when we went away for a long weekend (with our neighbour popping in twice daily to feed her) she decided to show her displeasure by not using a her litter try and leaving us a little puddle of wee behind our freestanding bathtub.

She didn't repeat this for a while as we didn't go away again, however this last week (a long time on since the first episode) she has taken to using this location as a permanent second toilet (still poos in litter tray tho, thankfully).
What can I do to (a) stop her weeing there? and (b) get to the bottom of why she's doing it in the first place? We have had a neighbour's cat get into the house recently on a couple of occasions, which she really hasn't liked AT ALL, so I wonder if this is some sort of territorial marking she's doing.

Can anyone advise?

Cats often like to have two trays. One to pee and one to poo. The first thing I would do is provide another tray and see if that helps.

Did intruder cat get in through the cat-flap? If so, change it to a microchip activated type, then only your girl will have access to it.

Cleaning the pissy area with bio washing powder or specialised cleaning product will remove the smell, thus discouraging her from going there again.

No, the intruder got in through an open window (we live in a flat). Our cat refuses to use the cat flap, or even go outside, but likes perching on the outdoor window sill and squeaking at birds, hence the window having to be open. Neighbour's cat is very friendly and inquisitive, and will come in at every opportunity; we try and monitor/prevent him, but inevitably he sometimes gets in as we can't spend all our time sitting with her fending off unwanted guests!

Thanks for the bio washing powder tip anyway - have tried that, and will see if we have a repeat performance tomorrow or not.

Thecatneuterer - do you have any idea why she might suddenly be doing this? Perhaps I should take her to the vet for an investigation. Neighbour thought it might signify a urine infection or something..