How did Jonas know that the leaders of the community were starting to give up the search?

When Jonas first left his community with little Gabriel, they frequently had to hide from low flying search planes.  Jonas counted on the fact that the pilots could not see color and that he had transferred memories of winter to himself and Gabriel so that the sensors on the plane could not detect their body warmth.  These planes usually came during the daytime, but they sometimes searched at night.  

As time went on, Jonas...

When Jonas first left his community with little Gabriel, they frequently had to hide from low flying search planes.  Jonas counted on the fact that the pilots could not see color and that he had transferred memories of winter to himself and Gabriel so that the sensors on the plane could not detect their body warmth.  These planes usually came during the daytime, but they sometimes searched at night.  

As time went on, Jonas noted that there was no longer any sign of human life and "the frequency of the planes diminished."  When a rare plane did fly overhead, it flew more quickly, as if the pilots were not searching as thoroughly as they had been before.  It seemed that they had almost given up the search for Jonas and Gabriel.  Then at last Jonas did not see any planes all day and all night.