Drones are fairly loud, you can hear them unless they are a few hundred feet up if your hearing isn't damaged and there isn't a lot of ambient noise. In this case the drone is tens of feet up, so they could see and hear it.

With how poorly trained and how they are kept in a state of fatigue, the Russians have two options; wait to die and end their misery or run and die tired.

You can see the guy in the background just sitting there waiting for his turn. He isn't shooting at the drone, he is just waiting for one hell to end.