After spending a lot of time in the woods, IMO, we're not very perceptive, especially when it comes to things we're not looking for. In addition, it really is difficult to see things through trees. There's always movement going on in the woods, even after the leaves have fallen; branches moving, fallen leaves blowing, water rushing, etc. To someone not looking for people, I think someone walking, especially at a distance would blend in with the movement and just register as static/noise. There would be breaks as people moved behind obstacles (trees, rocks, bends, etc), so at quick glance, they may not be visible.
Looking at the map, it looks as though the bridge would have been the best possible place to see BG and the girls if the timing was right. But it's likely the worst place to take your eyes off of your path. I also don't know how common it is for trail walkers to meander off the trail and down the creek. If it's fairly common, it might not have registered even if someone was spotted. The people under the bridge were likely spotted because of proximity.
Snipped for space- I think BG could have been a decent ways away and heard a car door shut. For some reason, that sound seems to travel well through the woods (it also makes the roosting turkeys gobble). And being teenage girls, it's quite possible they could have been talking or laughing and been heard by someone in the distance. I don't think he needed to be on the trail at all to hear or see them.
I'm interested in the Hoosier Harvestore/U-store. I believe the home marked as the Harvestore might be the office, but I believe the long metal building next to the shorter building about midway between the Nature Preserve marker and 25 is likely where things are stored. I'd be interested to know from locals if this is correct, and if so, how much traffic it gets on a daily basis. It would be easy to see from the buildings to the pull off/ parking area by the trail.
While I can't answer with any certainty, my guess is someone screaming might not even be noticed, as sad as I am to say that. Depending on how far away the closest people were to where the girls were killed, it may have been hard to hear anyways. There appears to be a bend in the creek near the CS that might absorb some sound or cause it not to travel well towards the bridge. I'm sure there was also the sound of the creek.
Or someone did hear something and just hasn't made it known to the public.
Or there was no screaming at all. Shock and terror can make people do unpredictable things, like freezing instead of running, staring wide eyed instead of reacting, quietly sobbing instead of screaming.
