It wasn’t only the darkness that made it impossible to see the boy dropping the gun, he flipped it behind the fence as he turned, the toss hidden by his body and the fence. He chose that fence line as his stopping point, too clever by half.
His arm and body moved in one continuous motion , the same motion he would have used to bring the gun to bear on the officer. I wouldn’t fault the officer in this case, and there are many cases where I do, Chauvin for instance.
Look for the Chauvin video with his knee actually on the shoulder blade.