Family have stated they were to be picked up after 3PM, DG showed up around 3:12 or so at the pick up site.
Judging by all of the information shared over the last almost two years, the 2nd SC pic was uploaded at 2:07. A young searcher showed it on his phone to a reporter that evening. My theory is BG hung back closer to the parking area than to the bridge itself, there's a slight bend in the trail along there. When he figured no one else was around he made his way towards the bridge, from the bend area both trails would have been easily seen, after all it was February and the leaves were down. So he would have seen the portion of the trail which comes from the Freedom Bridge, the part of the trail leading to the MHB, and the trail which leads down to the creek, through the nature preserve.
If I were to apply a time frame of being on the bridge and crossing it towards the girls it would be 2:17-ish to 2:23 or so, it would take a few minutes to get to the bridge from the bend, then another 5 minutes to cross the bridge, give or take. Again, guessing, he may have been within sight of the bridge at some point, on the trail, when A&L were crossing it, then at some point they realized he was headed towards them on the bridge.
Timing was key, here.
JMO