When Montag returns to the firehouse, where is the mechanical hound?

The Hound was gone. Beatty had sent it to Montag's house,

"The Mechanical Hound was gone. Its kennel was empty and the firehouse stood all about in plaster silence ... and Montag came in through the silence and touched the brass pole and slid up in the dark air, looking back at the deserted kennel, his heart beating, pausing, beating."