If the drone had it's transponder on, the track is easily available. If it had the transponder off, it was up to no good and a threat which should not be tolerated.
This is a spy plane, trying to spy on a hostile foreign nation. Of course the plane had its transponder off and of course it was up to no good. Spy planes/drones, well spy. if it was broadcasting its location, Iran would have been tipped off they were being watched, and their radio signals possibly intercepted, and would have the opportunity to try to impose countermeasures against being spied on.
A plane flying over international waters without a transponder on is not a threat and is free to fly under international law. Shooting down a plane traveling in international airspace is an act of war; this is true if the plane had its transponder on or off.