I've been that person so I would know. It either means you have a bat in the cave and she doesn't want to embarrass you, or, more likely, even though you broke up, she still likes you and can't take her eyes off you, but she is too afraid to confront her feelings and talk to you. Trust me, been there, done that, and if you tell her you are aware of it, and you don't want to get back together, she'll be crushed. I guess either ignore it, or if you do tell her, don't be a douche and play the still-friends card unless you really mean it. Though I'm not an expert on love, so you may want a second opinion.