I'm not sure what the proper facial expression is to give in return.
Depends on what you want to happen next. If you want it to die right then and there, then I suspect you've already got the right look on your face. If you want to explore the possibilites, then...
When you catch her looking, look back, not overtly, as turning your head toward her, but kind of looking sideways; but be sure to make eye contact. Do not lose the eye contact! Let a smile begin to slowly appear. Not a big grin, just a subtle hint that you like what you see. Let the eye contact last longer than it normally would if you and somebody happened to glance at each other at the same time. Then, after a suitable time, look away. Then immediately look back again. Now, grin.
COMF
Some for the glories of this world; and some
Sigh for the prophet's paradise to come;
Ah, take the cash, and let the credit go,
Nor heed the rumble of a distant drum!