My experience is only with consumer drones, but you could fly over a target area and release an explosive before anyone heard that it was there, especially in a

My experience is only with consumer drones, but you could fly over a target area and release an explosive before anyone heard that it was there, especially in a noisy environment. Above 100m, unless you're at high speed/power, most people won't notice a drone at all. It's often a change in speed/direction that gives them away, otherwise it will be past you before you first notice the sound.