LE has gone through the girls computers and socially media accounts, lets say rather thoroughly. But. What if someone else at the bridge that day had thought they were meeting a teenage boy and it turned out to be an adult male. We know that at least one of the girls' classmates was there at that time, and it was a popular spot for their peers. A girl makes a plan with a "teenage" boy, doesn't see him, and leaves, or maybe never shows up at all. BG is there to meet up with a young teen, and is enraged at being stood up. Its at least as plausible as there being a puppy in his coat.
I like this theory because in a sense it rules out coincidence, in other words BG just being there at the right place and the right time. It has always seemed too far-fetched to me, such a long shot, that all things came together on the day in such a short time. So Is it possible that a local girl from the school approached LE, further down the line of the investigation, and admitted that she had been arranging to hook up with an online 'date' on that particular day, in that same area, but for some reason didn't show up. So he arrives and is waiting and only sees Libby and Abby. Maybe he did think one of them was his 'date' and he was angry that she brought someone....
And so perhaps it was this other girl's computer/device that was researched and from that point on, LE lured that 'date' in, over many months, and Hey Presto, we have the arrest of a sexual deviant. It may not be CE but there might be something like this going on in the background that we, the public won't hear of until all loose ends are tied up..
I listen to a FBI case review podcast with Jerri Wiiliams and these retired agents discuss solved cases, and it's amazing how long it can often take to lure criminals in, through undercover work or using cooperating witnesses. These things take a lot of time.
Just to emphasize, I'm saying this all as a THEORY and just to back up what the above quote suggests, so JMO!
Remember in one of the first LE press statements, there was a statement like "Parents, know what your kids are up to' or words to that effect.