The small drones (<3kg) are quite capable of flying for 15 minutes or more. At 30-40 mph, that gives them a range of 8-10 miles round trip and are very quiet. They can be programmed for flight paths using GPS with very inexpensive equipment. They can also be programmed on loss of signal to fly to a particular altitude and return to home. They are a huge concern for counter-terrorist operations in the US. Even a small one can carry a deadly package of C4. Just program the flightpath, let it go and drive away. They've been operated without permission near roller coasters and almost anywhere else you can think of. You could imagine a passenger on a roller coaster smacking into a drone or an aircraft ingesting one. No wonder farmers are increasingly hesitent to allow anyone on their property.