Well they should do...otherwise what's the point of having codes.
I'm not quite sure what you're saying. You could still have a second hand RNS/RCD unit legitimately, although lets face it most aren't.
For instance your mate sells you his RNS510 out of his Mk6 which he specced from factory, but then chooses to upgrade to an aftermarket unit, then realises he has lost his code card. That was the scenario I was trying to manipulate.