One theory that I have regarding the killer being alerted to the girls presence on the bridge that day:
KAK listed (at some point) that he was the owner of Kline Photography. It could be that under the guise of Anthony Shots, he told one of them that one of his favorite spots to take photos was the Monon High Bridge and that he planned to be there at some point that day.
We all know that it has been said that the girls had no plans to go to the bridge that day, that they had planned a Mall trip that was apparently pushed back to late afternoon or early evening. They were bored with the hours that lay in between.
Maybe one of the girls recalled the fact that the Shots profile told them he would likely be there and they thought "Maybe we will actually see him out there and get the chance to meet him."
Whoever it was that killed the girls that day COULD have offered this scenario to several different girls HOPING that they would "bite" and show up. I remember one individual stating that they saw who they thought BG was and said hello, but that he just glared at her. It made her feel creeped out. I do believe she was with a group of others, and that could be what saved her. (all just speculation).
Anyhow- if he was there- and this is what occurred- that photo of Abby on the bridge that uploaded to Snap Chat is all he would have needed to understand EXACTLY where they were. It fell into place, just as he had hoped.
This all just my opinion and speculation , and one of several things that I believe could have happened.
MOO JMO AMOO