ADS-B Coverage
Automatic dependent surveillance – broadcast. Automatic dependent surveillance – broadcast (ADS–B) is a surveillance technology in which an aircraft determines its position via satellite navigation and periodically broadcasts it, enabling it to be tracked.
This map indicates the extent of ADS_B global coverage. Any aircraft flying outside of the green areas will not show up on the various tracking sites, applications e.g. ADS-B Exchange, Radarbox, Flightradar24, etc.
If you have the reg or flight number etc you can check the plane's flight history to track where it has been. There may be a delay of hours or days before the flight history is updated so if the interesting flight does not show up immediately make a note of the details and check up on its history a day or two later.