Can you describe the type of glance? Like, negative or positive? Seductive, admiration or just observing type?
Is your hubby quite good looking?
There are many reasons why she's doing that.... are you suspecting something?
I can see why you're disturbed (if you are), but it will be just as disturbing if she 'glances' at you too.
Sometimes people are just weird in their behaviors, un-explainable.