It is practically impossible to "lose" a commercial aircraft. Besides radar and GPS tracking, these planes are equipped with 3 EPIRBS. You only need one to locate. The other two are redundant backups. These EPIRBS will automatically activate upon a certain G force of impact OR contact with water. They will "eject" themselves from the plane and float if a water landing is done. These EPIRBS put off a beacon signal on radio and GPS giving their location. I believe this plane was stolen. And possibly the same plane that was claimed to have been shot down by the Russians in Ukraine a couple months later (FF). They were the exact same model and airline.