Originally Posted by mozilla

Despite the helicopter not broadcasting ADS-B data, it was equipped with a transponder that was actively responding to FAA radar interrogations, as mandated in this particular airspace. Unlike ADS-B, these transponder responses do not include location data, but they do provide crucial ID and altitude information. The FAA then queries its radar systems to determine the actual location.

Aircraft tracking sites, such as FlightAware, do not operate a radar. Instead, they depend on numerous ground stations (run by volunteers) that receive ADS-B data and transponder responses, which they then transmit to FlightAware. As long as there are sufficient ground stations in the area, FlightAware can use an algorithm to calculate an approximate location, which is less precise than ADS-B, by leveraging the known exact location of the ground station and the precise time that transponder responses arrive. This is referred to as Transponder Multilateration (MLAT).

Unfortunately, despite the safety risks involved with a shift towards less transparency, FlightRadar24 and FlightAware agreed to voluntary FAA requests to hide data from certain planes from their systems. This includes the helicopter in this incident. So that's why you won't see it on their websites. ADSBXchange did not agree to do this and continues to show everything as is perfectly legal.