In the context of the Drone Incident Management Manual for Aerodromes, the sighting of a suspected drone becomes a drone incident, when the verified drone has no clearance and has moved into or is present inside the flight-restricted zone (possibly declared as UAS geographical zone) and is therefore located in one of the “threat zones” at or around the aerodrome for which the drone is not authorized.
Reference- Drone Incident Management at Aerodromes on C-UAS Hub or EASA