Every time I try to figure out where this happened, I come back to the area around the Collin's home.
I've read varying times that the girls first went for a bike ride, so it was somewhere between 11:30 and 12. We know that the girls remained close to home unti 12:15, per the video camera. There are apparently two other sightings that may or may not be valid. One is on Brovan Blvd (in front of the house) at 12:23 and the other is at Gilbert and Elmer between 12:30 and 1 PM.
If the girls were riding bikes near the house for at least 15 minutes, doing exactly what they were allowed to do, why would they suddenly - shortly before they were expected to go home, decide to ride to Meyers Lake, park their bikes at a drainage gate, go through the gate, leave the purse on the ground, and disappear? That just doesn't add up for me. First, if they wanted to take off to the lake, why didn't they do it right away when they started their bike ride - why wait at least 15-23 minutes? Secondly, if they wanted to go to the lake, why were they riding towards River Forest Road when several other roads are safer and faster? Third, if they decided to take off on their bikes knowing that Wylma had to leave for an appointment soon, why would they park their bikes, go through the gate, put down the purse and do anything ... that is, a quick ride to the park and back is one thing, but the bikes/purse give the impression that they were dawdling, exploring, passing the time without concern for the consequences.
Because of the above points, I am inclined to think that they were taken at the corner of Brovan Blvd and River Forest Road and that someone carefully and deliberately staged the bikes and purse based on the assumption that the girls would not be missed for some time ... that when the bikes and purse were found, everyone would assume they were on an afternoon bike ride and happened to stop at that location to explore the area.