Okay, so I'll throw out something different (and probably extremely implausible).
So let's say BG was parked at the CPS lot, which LE has put a vehicle there as late as 5pm. If we say the girls were killed around 2:30-3pm (which could be wrong), then what was BG doing for 2 hours? There could be countless things, but here's just one thought. I'm going on the idea he didn't cross the creek, and didn't want to. Being wet would only make him uncomfortable and noticeable.
So LE initially was asking if anyone had been seen walking Hoosier Heartland hwy. What if BG took a roundabout way to get to the highway, with the least chance of being seen until he was on the highway itself. Then it could look like he was coming from the exact opposite direction of the crime when he headed back towards the parking lot.
I mapped out different routes through the woods and fields and came up with between 3 and a half to 4 miles, which comes out to be 2 hours or less. If he went south, he also could have followed a route through the woods and come up with the same, but I'll only post the north route because I'm probably insanely incorrect and nobody will care, anyway.