Civy number plate, tax disc, blackout, dark clothing, could she be AFS or NFS? I'm not suggesting she would be sent to put out small fires, but they did use messengers.
They were certainly used in the early post Dunkirk period on impressed machines but I suspect the Army standardised with the regular hooded type very soon.
Maybe its wrong to call an NFS bike civy, its still a government department with equipment issue organized by the Ministry of supply.