dog detects low blood sugars in owner

A client came in yesterday with his little chihuahua for vaccination and he told me that this dog was pretty special because it warns him that he has low blood sugars (hypoglycemia) which is a very useful thing if you are diabetic. He told me that the dog regularly smells him and if the dog thinks he is low, the dog taps the owner on the foot and growls at him until he goes and eats some jelly beans. I think this is amazing, he never taught the dog this. The dog just fiqured this out. My Wife is diabetic but our two dogs have no idea about her low blood sugars. What gave this dog the ability to do this? The only difference I can see is that the dog is the only and constant companion for this man and they have developed a very special bond, still this dog must have a special insight or intelligence to have worked this out. Meeting people and pets like above pair really make my day as a vet.