Oh wow!! This is similar to the situation that I’m in. Stares been going on for a year or so, don’t work with the person but we are in the same building. He just stares a lot, I use to smile when I’ve caught him but a smile was never returned. Even said hi and again nothing. And I’m questioning myself as to why he stares everything I see him haha.
So I believe that she really fancies you but maybe isn’t too sure as to how you feel hence the staring you out (which is creeping you out) and also she is just taking your presence/beauty in.
It’s possible that she is approachable she could just have her guard up, she may not be sure as to how you feel, or asking herself why is he staring at me? Hence why she stares at you with no expression.
Since you have both spoke before why not strike up a conversation with her? Start with a smile and ask something like how’s work? Haven’t seen you in awhile? Etc. Start slow and see how it goes from there.
Hope it works out for you!!!