Does covering one's face in public present a security risk?

No, certainly it does not represent a security risk when someone is covering the face in public. It can have different implications. For example, if women is covering her face in public, it could be due to her own choice regarding the religious paradigm. It depends entirely on her how she is interpreting the religion and what she thinks best for her. It can also happen that sometimes covering of the face might be to hide her identity from the public. But if man is covering his face in public, it could be due to save his face from the dust, weather conditions etc but definitely not for the religious sake. In some cases, if man is covering his face, as we see in the movies, it could be for some security risk perspective.