I was also curious about that ... about how the situation was perceived at first. It sounds like police located the bikes, most likely picked them up and leaned them against the fence to get them out of the way. Then they looked around for the girls. It seems like they assumed that they wandered away, especially with the two weekend 1000 people searching the 12 acres. Wasn't it after this that the drowning theory kicked in, even though the purse had been found on the lakeside of the fence. The lake draining took another 5 days, a little longer than expected. Finally, it was labelled an abduction. Although there were parallel investiations ongoing, much of the scene was contaminated while the drowning or lost theories were excluded.
It's understandable that the first assumptions would be that they simply wandered off (benefit of the doubt), that they got lost and needed to be found. After exploring that option, the next optimistic option was drowning. Finally, the worst case scenario had to be acknowledged.
If the bikes were staged to obscure evidence, it worked.