Hi,
Dogs in the UK and the US are now being trained to detect the onset of epileptic seizures and diabetic hyper and hypo situations. Many people have reported their dogs reaction to their illness or condition but it is only fairly recently that such dogs have had this "sixth sense" exploited and encouraged.
It is, however, still a mystery how these dogs detect something amiss. Theories include subtle physical changes in the owner such as tiny tremors or a slight increase in sweat production not detectable by the owner or other people or subtle changes in body odour.
Whatever the signs to the dog it is another example of how much we still have to learn about these truly incredible creatures.