Drones are incredibly cheap. Why was not all surveilled with drones? The close areas should have had agents on the buildings especially the one most close to Trump of which the assassin tried to kill him. What happened is total incompetence or it was worse I pray it wasn’t something worse.
Drones with long loiter times are less cheap, but still available, especially ones with wings. If they put up several, then operators become an issue, however they can also be pre-programmed like those light shows, or a simple orbit around a spot at an altitude.