I think a ball being thrown would get the attention of any horse that was awake. It would certainly spook all of mine. It's certainly not something I would worry about having happen on trail unless your rides frequently go through sports stadiums. ;~)
The key to staying safe around that kind of thing is not desensitizing the horse to everything they ever spook at, but being alert to possible outcomes, like the ball bouncing toward you, or a dog, before they happen and keeping a safe distance.
I think I figured out why she was bumping that leg when I was leading her. No eye rolling, ok, but I believe it was because I was in her space. Today, I stayed to the outside, instead of stepping over it with her. She was perfect. Now I just need to figure what I'm doing wrong in the saddle. Maybe, I'm not giving her, enough slack. She can do it so it may be justing a cueing thing.