New kitten toileting everywhere

[Nahhh]

We have a new female kitten who is 9 weeks old. We have an 8 month old neutered male kitten too. New female kitten is kept in the front room away from the older kitten. She has her toys, food and two separate litter trays in there. We also have a feliway plug in.
However, she is pooing all over the carpet. I’ve been cleaning up with an enzyme cleaner especially for cat mess. I found the room was smelly so I just pulled out the sofa and she’s also done loads of poos under there.
She will and does use the litter tray but for some reason is also pooing all over the floor.
Can anyone advise?

Oh goodness!

Are you cleaning the tray out often enough? Some cats really don't like using one if its dirty.
Is the tray in a place where the kitten feels safe?

I remove any poos as soon as they happen and am emptying it completely (except a little amount of dirty litter) every other day.
Both trays are in quiet corners of the room and she does seem happy to use them but is then doing these sneaky poos when you don’t realise.

They're not covered trays are they? I read the other day that cats actually dislike them as they feel trapped at their most vulnerable.

May a vet check is in order as that is quite unusual behaviour for a cat?

No they’re open.

She’s going to the vets tomorrow anyway so I will ask then.

Talk to your vet. This is most likely inappropriate illumination that can that can be easily corrected.

Thanks, what is inappropriate illumination and how does one go about fixing it?