How does Cole signal Jack to alert her to the fact that he is approaching the cottage in the novel, Zero Days?

When Jack hears a car approach the isolated cottage, she is afraid that she has been discovered, so she gathers her belongings and hides in the brush. Soon, however, she hears the person whistling the opening riff from the song that she and Gabe danced to at their wedding and realizes that the person is Cole, and she is in no danger.