I am wondering about BG :
1) No one (that LE has told us about) saw him, even though there were other people in the park that day. That tells me that he was not there as a casual walker, but deliberately hid himself from other hikers. He probably did not use the trail at all until he got onto the bridge. As you hikers know, you can walk along a trail like the one in Delphi and not see a person or a forest animal that is 10 yards off to one side of the trail and keeping still. Especially if you're talking to someone while you're walking.
2) No one at the park heard him or the girls after they encountered him. This would be understandable if he had either disabled their voices right away (tape) or threatened them with something like a gun. That tells me that he came prepared. His pockets, and possibly his jacket, were not empty.
3) No one has seen him bearing signs of a fight with Libby (per her grandfather), at least not in the immediate aftermath of the crime. That tells me that (assuming her grandfather knew with certainty that she had fought the perp) he either lives alone, or with an older parent(s), or a really dense partner.
4) We know that he was, at least for part of the time, on private property. He must have known that it was private, due to signs and fences. What's the first thing you would think if you were trespassing on rural private property? The owner's dogs. If the owner(s) had one or more dogs with good instincts, you'd be in trouble. The owner could be in his bathtub at the time, but the dogs would be on you in a second. This might tell us that he knew darn well who in the area had what type of dog, if any, and if the dogs were typically outside. Of course, LE hasn't told us if any of the dogs close to the trail had alerted to BG.
5) If BG has ever been to a doctor or a dentist, it stands to reason that one of those people would recognize him. Maybe. Besides, where I live, every one of the medical/dental offices takes your picture before you are seen for the first time. It would be sweet if he had to go get medical or dental help now.
6) His driver's license picture. I wonder if the Indiana DMV saves those digitally? If they did, they could do a comparison.
Just my random mental output. And MOO.