Had it occurred through Libby's phone, yes. And apparently LE has been through that phone and any family computers. (Does their school have a computer lab or classrooms where they may not necessarily be assigned a specific computer? Just thinking out loud as I don't know the answer.)But wait a minute, wouldn't there be proof of a planned meeting/chat/convo re on Libby's phone that forensics would have found?
Of course, a forensic analysis of phones and computers will not detect the old fashioned method of communication. Personal face-to-face conversation. Had there been a conversation the week before in the hallway at school about what to do on the day off and word of mouth got to one person and then another is certainly a possibility. Yes, family members said it was a spur of the moment thing, but family doesn't always know everything.
With today's society - not just teenagers like Abigail and Liberty - odds are it is in social media or a text. And it sounds like LE has gone through all of it.