They may not even have got out of their car.
Remember, they were expecting the girls to be there...not to NOT be there, if you know what I mean.
By this time they were probably starting to get alarmed and I can easily imagine they thought a drive by was the most effective and quick way of checking. The lake looks fairly open view-wise, the family knew they had their bikes, so they would have been looking for two girls on their bikes, not two missing girls and two discarded bikes.
I too would've driven by slowly....nup, can't see em...where else could they be? Let's drive past x....they might've gone there....and kept on driving, probably all the time saying "those little brats will be at home watching tv when we get back".
If they were expecting to find two girls, on their bikes, already late and in trouble at home, they would be expecting to find them on their bikes, pedalling like mad, somewhere on the road home. All they needed to do, in their minds, was find the right road, and they would find the girls. It is absolutely natural and understandable they didn't get out and look.
IMO of course.
:moo: