Criminals stage crime scenes all the time. . .to throw off where the crime actually happened, or to hide evidence from people in their lives. One couldn't have the girls' bikes in their garage or work van. So a perp may have a need to get rid of the bikes in an area that wouldn't give away any details as to what route they headed with the girls.
Where the girls were actually abducted or where they were taken, may give clues to who the perp actually is. Unfortunately, in this case, if the bikes were staged, it seems to have worked.
Think of the planning that would go into something like that ... hard to say if they're still alive. If they are, it's not good ... I'm thinking worst case scenario, like the Long Island girls. They were kept alive for some time.
Clearly, if the abduction occurred at the parking lot, the bikes had to be hidden ... what better place than one that will stall the investigation. If the abduction occurred at the lake, how perfect that they met at such a secluded location.