Ok, this might be really off-the-wall, but I'll say it anyway.
Is it possible that the bikes seen at 12:27 weren't those belonging to the girls? If they were in the middle of the path, and nobody admits to moving them, could they have been placed there by someone waiting for someone vulnerable to stop and check them out or move them?
I keep thinking of someone (maybe at least one a woman or teen so they aren't so intimidating) could have called to them when they stopped with some kind of a ruse - puppies, kittens, needing help, whatever. Once the girls got over there, they could grab them, throw them and their own bikes in the back (of a van, maybe?) and left. It could have been any time between 12:27 when two bikes were seen and the time when the girls' bikes were found. That could also account for the sighting later in the day and the bikes being there earlier.
Or, maybe I've watch too many mystery movies...