Dogs hear in a range far above the capabilities of the human ear. The silent dog whistle emits an ultrasonic blast, where we only hear the sound of rushing air. F-4 Phantoms were taking off and you can not hear anything else. The handler could see the dog cock his ear to better listen to the offending noise. That action warned the handler.
Dogs have superior night vision.
Dogs could see movement long before the handler could. The alert was simple to recognize, when your partner starts staring in one direction, it's because there is something to see. Of course we could stare in the same direction and only get a headache. That's the facts, dogs can see, hear, and smell bad guys or bad things before we can. Now granted, some maintenance is required and a dog will not work non-stop.