Wondering if he was accompanied there by like a father, uncle, brother figure who had to suddenly leave and told him to stay there, he'll be right back. But it is obvious from the boys comments that whomever were not "right back". Took quite awhile it seems, the type of time probably, 4 - 8hrs, that would be necessary to get the situation under control for this type of crime.
Age and times would be nice as the parents could chime in about him being left there all day, unattended.
Verification of if he traveled alone of with someone is needed? Did he walk? Bike? Picked up? Dropped off?
If he neither saw nor heard those girls, and he was there all day, than I believe the girls never made it to the lake. Bikes were placed at the lake which would corroborate their scent being there. This however would cast serious suspicion IMO on the bike rider's story who had to swerve to avoid the bikes in the path. This would imply the bikes where placed in the pathway, than moved by the lake. No UNSUB(S) in their right minds would return to a crime scene TWICE to move bikes. Only other way was that the crime was in progress. Which the biker would of ridden right by. In saying that I see only two possible scenarios:
A. UNSUBS moved the bikes twice. Crime in progress or return to scene.
B. Bike riders story is false and he should be further scrutinized.
The bikes were either in the path or not in the path. If what the bike riders said is true, than the UNSUBS moved the bikes twice or the crime was in progress. Boy fishing nearby hears nothing. Figure in statistics and odds of this happening, it just seems odd. IMO
***Assuming abduction either at the lake and bikes placed or before the lake, bikes placed at lake.