Almost all aircraft are equipped with ADS-B transponders, which transmit their tail number and location continuously. Anybody can buy a ADS-B receiver and see

Almost all aircraft are equipped with ADS-B transponders, which transmit their tail number and location continuously. Anybody can buy a ADS-B receiver and see info for aircraft flying near them. In good internet fashion, people figured out that they can coordinate large networks of receivers, which basically centralizes a nationwide database of all aircraft movement. So, all this account did was tweet when Elon's jet moved from this central ADS-B database.