I always taught that it was good practice to say in what way the service is limited. For instance if the aircraft is near the edge of the display and the bottom of radar cover, the controller might say "Limited Radar Information Service, little or no warning of traffic from below or from the South". However I would suggest that a large number of unknown returns is not a reason for providing a limited service, the service should be downgraded to a FIS. There is no such animal as a limited FIS. Unless of course the rules of the game have changed.