I think you should say something. If your choices are say nothing and feel bad thinking she didn't value you enough or letting her know you heard she was moving and you want to say bye (in a positive way) ..... I would choose the chance to say bye. That lets HER know that you valued her enough to say bye and as a previous poster said, maybe there's a reason she hasn't yet (or no really good reason - it could be that the move abroad is overwhelming her).