I'm not aware of any retail drones with more than 20 to 25 minutes of flight time. Many will get 1200 to 2000 feet out of the box, but there's plenty of users who've modified their uav's to get 2 to 4 miles.
DJI and 3DR have flight software that allows you to set a course for the uav. DJI's software limits the distance to 1500 feet, but I'm sure someone knowledgeable could eliminate that restriction.
There's all sorts of drone sensors available, which listen for characteristic noises. I've read that drones on radar look like birds, although I would think a bird flying straight for a couple of miles might be unusual.
I don't know how much coke the uav could carry. I've seen videos of the latest DJI Phantom 3 lifting 2 pounds, but that's just hovering a few feet off the ground. There's another uav, the Spreading Wings, that will lift 8 pounds. Both would have their flight times severely shorted with that kind of weight.