Honestly it sounds like it could be E-Cuniculi, This requires a special blood test, but that often comes up as a false negative, so it’s a very unreliable test. A positive is only shown in bunnies in the top tiers of the infection. Kidney issues are common with EC as it tends to affects the kidneys and brain/neurological functions. My bunny Bombur has chronic EC and kidney issues related to it, and peeing himself and his cage was the only sign we really had that something was wrong. We assumed at first it was behavioral because, like you, he didn’t pee outside the box when he had play time. He also had been urinating and drinking excessively, and still does during a really bad flare up.

You can ask the vet to treat with a standard 28 day course Panacur or Oxibendazole if EC is a possibility. The drug won’t do any harm and will only help him if he does have EC. The drug takes a week or two to fully take affect, so you wouldn’t see immediate results. The drugs can also cause low white blood cell levels, so you’d need to have a blood test at 2 and 4 weeks of treatment to make sure everything is in order.

It could still be that he cannot control it, so when he feels it coming, he’s simply going back to the cage instead.

Have you noticed an increase in water intake? I would also consider the flooring of the cage, and seeing if you can expand the litterbox and/or put more absorbent things in the cage so it’s not as wet.

Bombur only has the urinary symptoms and some mild eye sight issues. Other than that you’d never know he had it. He tested positive for it mainly because his counters were so high. Once he’s on treatment for a month he’s good to go until his next 3 months. Bombur has a chronic case of it though, whereas most buns will likely just have one or two flare ups in their life. You could still ask to test for a UTI or treat with antibiotics, but if those don’t work I suggest asking for a 28 day course of panacur to be safe. When it’s just urinary symptoms sometimes vets won’t catch it or notice it because EC is usually associated with head tilt and things like that. Peeing and excessive drinking does seems like there is something going on.