A rabbit can pee in his “sleep” if he’s super comfy and resting and being pet, ie in a stste of deep relaxation. My bun Bam could do that sometimes, especially when he was lying in my mom’s lap -he jumped up on her lap himself, so he wasn’t restrained or stressed in any way. He could also sometimes pee on my couch when he and I were cuddling.
Many rabbits pee on soft surfaces like beds, couches, pet beds, pillows etc.
If a rabbit pees himself in his sleep Id be worried though. If they are unwell they can sometimes wet themselves, but it’s rare. It happened with one of my buns once.
General incontinence (peeing small amounts of pee all over or dribbling pee) can be a sign of a UTI or e cuniculi.
If he seems otherwise fine, no signs of illness or disconfort etc, I guess all you can do is see if he’ll do it again. If he does, he probably will make a habit of it, so you’ll need to block him from accessing your bed.
I’ve had two buns who slept at the foot end of my bed and didn’t pee in it, although one of them I think peed in the bed just the one time. I’ve also had 2 buns that were completely hopeless on soft surfaces. I suppose it just varies between individuals. One of the non-peers wasn’t even neutered (health reasons).