Why does she always look down with a slight smile when she passes me?

There is this young woman at work, a year or 2 older than me (I am also a woman). We both have a fairly similar dress sense (reason why I notice her lol) and are the more younger ones at work compared to most others. Anyway, we do not work together but see each other around everyday. I have noticed that every time she passes me, from afar she acknowledges me (meaning she sees me), but as she gets closer and passes me, she avoids eye contact and always looks down with a slight smile. She also looks to the side whilst looking down, as if she is looking at my shoes (I do wear quite a few different shoes so not sure if this is why lol).

I am not sure if the slight smile whilst looking down is a smile for me or that it signifies she knows some secret about me or something negative?! I have not been able to tell if she does the same with other workers or not so can't say much on that. I am also not totally convinced that she is overly shy. I am an extremely shy/nervous person but still manage to make quick eye contact and smile at others if necessary. She also chooses to stand fairly close to me when in the same room and she has the option to not (still no eye contact made or conversation started). Why do you all think she does this? It doesn't happen with anyone else at my workplace!