Did the girls have their first encounter at the bench with him perhaps? They didn't do video or audio then. Maybe, they had a conversation, maybe ambiguous and cheeky. 20 minutes in the girls' timeline were unaccounted for, I remember, we said here on WS. Of course, RA had to say, he didn't see them, if this 20min.-meeting led to the double murder. And of course, the girls filmed him, when he followed them onto the bridge. They recognized by his clothing, that he was the man from the bench, and they noticed (being scared), he had broken the rule, not to enter the bridge, when another person was already walking the bridge.
Maybe, it all began there at the bench?
Speculation, MOO