Thin slightly hyperdense membranes paralleling the skull vault and cerebrum are a feature of chronic subdural hematomas, indeed pathologists rely on the detection of these membranes at autopsy to confirm chronic blood consistent with hygroma. The same can be used on CT or MRI, i.e. if extra-axial membranes are detected on imaging then there are definitely chronic subdural blood collections rather than just prominent CSF spaces or bland hygromas. In this case, they are probably the result of "overshunting".