Regarding BG approaching..if he was specifically targeting the girls, or if he was specifically targeting females, or if he was specifically targeting humans it would all depend on
how he got to the area and where his vehicle was parked, if he had one.
Lets say he is in his planning stages of locating a possible victim or victims, he would scout the area while most likely avoiding to be seen by other hikers. That may or may not be true. If he is not a local, he will be less caring about being seen IMO. In order to control who is approaching the bridge area - if that's how he operates- he would have to move from the NW end of the trail(s) towards the bridge. If he is not trying to "herd" his victims onto a particular location, then he would just follow someone and making sure, nobody is close behind him.
Look at the map. From the point the girls were dropped off to the NW end of the bridge (the end of the official trail and the beginning of the privately owned bridge) there is only one curvature he had to watch knowing if anyone was behind him/them:
(curve picture in map credit to GH video)
If BG is not a local he may or may not have known, that the bridge was not part of the trail system. Even less chance for folks to enter it. Also, he may or may not have known, that there was no school that day. We just don't know, who he was targeting and if.
Again, I would say he approached from the NW side and very possibly followed the girls after they had been dropped off. He could see now who was in front of him and who was or wasn't behind him.
ALL IMO
-Nin