I think it could have happened very simply. The girls at around 12:15:
"Let's ride to the lake and back fast before we have to go home!"
They are riding fast in the video. They got to the lake and were going to ride around it and out fast.
At the point where the bikes were found, someone got them to drop their bikes and come with him, either by calling urgently to them "Come and help me find my dog, kitty, baby, whatever" or threatening with a weapon. They dropped their bikes and went into the woods with him and he quickly subdued them, either with a knife to the throat of one, or hit them, knocked them out, whatever it was, it was quick and possibly violent. TG rides by, they are back in the undergrowth being kept quiet. Perp then takes them to a waiting vehicle and is gone. All in a minute or two.
The noise from the adjacent highway has been mentioned here several times, it would mask any noise made by movement in the undergrowth and also any outcries from the girls.
It happens, so fast no one notices. It even happens in places not hidden nearly as well as this pathway is at that point.
I think this person was there and saw them coming from across the lake. That gave him just enough time to get into place if he needed to move a little or get a knife out, whatever.
I do not believe it could have been planned as no one knew they would be there.
I believe this was a crime of opportunity. He had the opportunity and he took it. It happens.
...the only thing that I can't fit into this scenario is the purse being on the other side of the fence, set down neatly as if dropped over the fence and not thrown.
Where is the closest place that a vehicle could park to the spot where the bikes were found?
I've looked at the maps and can't tell. TIA MOO JMO