The distance varies due to wind speed and humidity. Now that may not seem great, but 50 yards at midnight, on a jungle post, during monsoon rains, is far better than tripping over someone that want to do you bodily harm. The fact is that the FAA has spent millions of dollars attempting to perfect a gadget to detect explosives in suitcases at airports. Dogs have been trained to do that since the early 1970's. Dogs have a superior sense of hearing.
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.