I'm not so sure. 40 minutes went by from the time that the girls were dropped off until 2:07PM when Libby took that snapchat which did not show the killer in it. So the chance of him seeing them enter the park and then waiting 20-30 minutes until he started following them is slim. If he were there just to stalk for victims it seems more likely he would be waiting by the entrance. Doesn't make sense for him to walk the trail or wait in the surrounding trees until the right opportunity walks by.
Also the fact that it was on a Monday. Going to a trail that was not busy doesn't make sense unless he knew kids were out of school. But then my next question would be how popular the trail was for school kids?
I just don't see it as a planned killing. I think he probably knew the trail. Probably fantasized about doing something for a long time and then the opportunity presented itself. Without knowing the murder weapon it's hard to say but everything thus far makes me think it was purely a compulsive decision.
My take on a timeline is the girls were dropped off around 1:40, maybe a little before. At some point Libby took the two images which were uploaded to SC, both were uploaded shortly after 2. At some point thereafter BG got on to the bridge and started walking towards them, my guess would be they were almost to the SE end of the bridge when this happened. From the NW end of the bridge to the spot in Libby's video would be at least a 4 minute walk at an OK pace, considering the condition of the bridge.
From 1:40, which is when I believe BG saw them being dropped off, to, say, 2:15 is 35 minutes. Plenty of time to get his ducks in a row to carry out the crimes he ended up doing, and to start that ball rolling.
I'm of the opinion he saw them being dropped off, and I can imagine what he witnessed. Two juveniles dropped off at the parking area right off of C.R. 300, right by the trailhead for the Girard Trail, so they don't have a vehicle with them. Perfect for what he wanted to do, vs. an adult or adults parking a vehicle which would later be seen by witnesses, searchers, etc.
I'm also of the opinion that the murders would not have happened had BG not seen the girls dropped off, had he been, say, further back towards the Freedom Bridge/IN 25 when they were dropped off, he would not have seen them, and there would have been no vehicle in the parking area indicating someone or some people had arrived since the last time he checked. The girls would have been around the bend on the trail, out of sight of the drop off point, or close to or on the bridge, again out of sight range of the drop off.
I picture BG loitering that day, waiting for just the right time to strike, the right victim or victims to attack plays into this. No vehicle parked there, but two juveniles end up on the trail and he saw them being dropped off. He had this all planned out, but A&L were the perfect victims, I think. There would be no indication of where they could possibly be when they went missing, as they were juveniles and thus did not have a vehicle with them. The only indication they got to as far as the bridge area was when searchers shared Libby's SnapChat uploads that evening.
JMO