Long-time lurker, first time commenter. I hereby affirm that the following statements are MOO.
I firmly believe the killer was someone the girls met online. Perhaps that trail was their prearranged meeting place, unbeknownst to the family member who dropped them off. When the girls realized he wasn't who he presented himself to be online, they tried to separate themselves from him but he arrived there with other plans and their fate was sealed. Evidence was recovered from Libby's phone, so we can assume it was accessible. But what about Abby's phone? What if her phone contains vital clues and conversations leading up to that meeting? If it's an iPhone with an unknown passcode, even the FBI wouldn't be able to unlock it.
Apple vs FBI: All you need to know
I only speak on this point because of similar things my friends and I did when we were young and naive 14 years ago as MySpace and "online profiles" became popular. Luckily we always met these "online friends" at the mall or other public places, but our parents had no idea what we were really up to when they were dropping us off. It makes my skin crawl to think about how trusting we were of complete strangers we "knew" from an online profile.
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Regarding "The Shack", I believe that's tied to a false alibi given to LE early on. "Can you account for ________'s whereabouts on the afternoon of the 13th?"... "Yes, he was at home with me"... "What did the two of you do that day?"... "We watched a movie"... "What movie?"... "'The Shack'". Perhaps LE is looking at the person who provided that alibi and saying, "You're full of $h!t and we know it. Come up off it."
The foregoing statements are MOO.