I am wondering what in this man prompted Libby to take his photo and start recording. I remember myself being young and nearly running into a garden-variety pervert on a bridge. What gave me the "danger" signal was not his face (he was far away) but the behavior (he hid behind a brick support).
What I want to say, if someone is far away from you, and you get this "danger" vibe, it is usually the person's behavior, and not the look. I wonder if the BG first approached the girls and walked by them, and then they saw him returning, for example. I don't have the feeling he was targeting them specifically, probably was on the prowl, looking for some random victim.