Where did you come up with that theory? I have not seen anything in the owner manual that would indicate those green charge LED lights are supposed to stay lit even when the car is unplugged. The manual does state that if the battery saver function was actually run, you would see a message in the instrument cluster and that it would only run for 20 minutes.
I have seen the same behavior as rosinchard but in my case I don't think it had anything to do with battery saver. It was probably just another of the Niro's many software bugs. In my case the instrument cluster was also stuck displaying 'P' and this lasted for over an hour after I unplugged the cable following a canceled charge. Most likely it would have lasted indefinitely so I manually reset the system by pressing lock on the remote.