Regarding BG leaving the phone, I agree he may have felt rushed or maybe was even spooked by something (heard a person walking on the trail or something?)
Also, though, I wonder if he figured it would be safer to leave any phones (not knowing it was actually recording him) so they couldn't trace/track the phone if he brought it with him. Most people have heard of the police being able to trace phones, and he may have just figured, leaving it there would only make it "ping" in the area of the trails and not possibly implicate him? MOO Of course, little did he know she was recording him! I believe when the attack began, she shoved the phone in her pocket and it was not visible to him.