From the documentation:
fetchOwner(key)
Return the operator which contains the stored key, or None if not found.
whilst
fetch(key, default, search=True, storeDefault=False)
Return an object from the OP storage dictionary. If the item is not found, and a default it supplied, it will be returned instead.
It would be nice if fetch would also return “None” if the item is not found and there is no default supplied.
If this is not possible, maybe the documentation should be amended to state that fetch() throws an error if the item is not found an no default is specified.