Thanks for your comments everyone!
I know peeing in their wheel is normal behaviour for some hammies (although Oreo has never peed in his wheels), just like it's normal behaviour for puppies to pee on the carpet
. I'm trying to find ways to encourage Bean to change that behaviour. The fact that he doesn't always do it gives me hope. I'm trying your suggestion, Merlina; I've put some pre-peed sand in a container right under his wheel.
We share a room, Bean and Oreo and I, and I have a very good sense of smell, so here's hoping!
Meanwhile, I'm glad that he didn't disappear while he was at large. I was surprised (but pleased!) when he came right out when I called him. SO glad I spent some time training him to come when called!