Seen pix of this woman? Reasonably tall, she looked, but still almost waifish, a wisp of a woman. How freaking hard could she HIT a car? Besides, based on everything we know about her, striking anything just does not fit.
She may have tapped the trunk of the car to let them know she was there so as not to startle them. If so she didn’t count on one of them being a paranoid lunatic.