I have seen a lot of these videos, and I admit that I have no idea how far away the drone, Apache or other aircraft might be. I am amazed at how oblivious these people are to what ever may be flying around, nighttime not withstanding.
We have a helicopter base about 25 mies away. Often choppers fly nearby. Blackhawks and Kiowas are not silent. I do not know what an Apache sounds like. I still can’t believe these idiots don’t hear anything.
Physics
Wikipedia states that in general the Hellfire missile’s range is ~8km.
UAVs are operating at medium altitude, typically around 15,000 feet. They are quiet, and very hard to spot with the naked eye. Moreover, both Predators and Reapers (the next-gen UAV) carry Hellfires. More than likely, the drone that shot them was the same one that spotted them. The sensor operator (an airman or NCO) detected the activity, and confirmed it with command element running the mission. Once confirmed, the drone pilot, sitting next to the sensor operator, was cleared hot, and you saw the rest.