In Fahrenheit 451 by Bradbury, what did Montag believe had been done to the hound?

Montag believed that the hound lost his trail at the river.

He was three hundred yards downstream when the Hound reached the river. Overhead the great racketing fans of the helicopters hovered. A storm of light fell upon the river and Montag dived under the great illumination as if the sun had broken the clouds. He felt the river pull him further on its way, into darkness. Then the lights switched back to the land, the helicopters swerved over the city again, as if they had picked up another trail. They were gone. The Hound was gone.