I was also under the impression that pups will leave their sleep area to wet/soil - Molo and his siblings were doing this at 4 weeks according to his breeder.
I am sure the experienced COL members will be able to help me here; could it be an infection of some sort? I had an adult GSDx bitch who occasionally got bladder infections and this made her wet her bed with wee in the night, and I'm sure she didn't know she was doing it.
Can this happen in dogs, and/or puppies, or is it more likely to be an immature bladder that needs time to develop?