Hmmm, I wonder if the foster kittens are triggering some territorial marking?
I feel your pain, I’ve had buns lose their litter box habits from time to time.
Usually the recommendation is litter box “boot camp”. Since they are using their box fine in their cage, restrict their space to the cage/pen for a little while, and then start slowly increasing the amount of space you give them (you might need another pen to help mark their area. You can also have them take “potty breaks” where you return them to their cage periodically to encourage them to use their box.
Since the extra litter pans didn’t seem to help, it’s your call as to whether to keep them or not.
I will also say that it helps mine to always have lots of enticing fluffy hay in the box to encourage them to jump in.
Have you actually seen them pee next to the box? Sometimes buns jump in, but then stick their bum over the side.
One other thing that helped me when my girl was peeing outside her box, was to mop it up with a paper towel and place it in her box, but then wait to actually clean it up until she was locked in her condo for the day. If I cleaned it in front of her, she would pee again right then! I also was originally using white vinegar to clean the area, but was using it so much that I think she was remarking to cover the vinegar smell. I switched to using Nature’s Miracle carpet shampoo (I have a little spot cleaner machine… life saver), and she stopped marking the spot!
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The answers provided in this discussion are for general guideline purposes only. The information is not intended to diagnose or treat your pet. Seek the advice of your veterinarian or a qualified behaviorist.