[Imogen]

Have you checked his wheel or hidey house? He may also be using his sand bath as a toilet. Usually hamsters decide to use one corner of their enclosure to pee in, so make sure you check each one. Hamsters are generally very clean and only use one place to pee in but they do have a few accidents and do pee else where besides their normal corner.

My hamster is toilet trained and goes in his little toilet in the corner of his enclosure that I bought off amazon to make it easier to clean (this saves having to remove a large chunk from the corner as it's a very thin layer of wood chipping he pees on and it then goes on to the plastic that I use a pet safe cleaning spray on. Saves it soaking down since he has 20cm inch substrate depth). But he does have the odd accident in his sand bath as well as his wheel.