Strawberry still does this sometimes, especially when in the dog crate. Im not sure why as he doesnt do it in his NIC set up. I guess maybe its because he feels more like the NIC is his territory as he spends more time there so has no need to mark.
If they havent been in the shed long it might be that it doesnt smell enough like them yet so they are marking it. If you could leave their litter tray for a little longer than you usually would it might help. (Obviously not too long that it will attract flies)