With his age in mind, it may be a sign to get him neutered some time soon. Since he likes the hideaway, he could be repeatedly marking it so it’s his, not anyone else’s.

If this is the case, you can wipe up a bit of urine with a small towel/old shirt, let it dry, then place it so it’s the flooring to the hideaway. He may not want to remark if he already has his scent on it. This is what I do with Wick until he’s old enough to get neutered, but he still likes to pee in non-litter places.

I agree with Wick’s advice: it’s probably hormone-related. A hormonal rabbit is usually a frustrated rabbit who’ll try anything to feel better. This varies from destroying everything in sight to mounting all objects with approximately the right size. Our bun frequently started to mount my knitting or crochet projects. Yuck! Good that you’re neutering him soon, most litter box problems are solved by it, and it’ll also prevent many behavioural issues. Usually the hormones will die down within 2-3 weeks after the operation.

Afterwards you can start to correct his litterbox habits if problems persist. One of the things to keep in mind is that rabbits love to pee on soft things for some reason. Your woolen sweater, the bed sheets, towels, their blankie. If you want to remove the smell of rabbit pee from something, wash it with white vinegar and rinse off the vinegar smell. Nobody knows exactly why, but sometimes the only way to keep them sanitary is to remove all softness from their surroundings except for their litter box. We had to do this to our bun as well, he’d use his whole cage when it was still covered with hay. Now it’s covered with cardboard except for his litter box which is filled with absorbent wooden pellets and hay. He uses it as a lounge-box as well, which isn’t a problem as long as you clean it frequently.

Best of luck with your bun and welcome to the forum!