I have a 18 month old patterdale terrier, she has always had slight bald patches behind her ears, which the vet told us was just because of how her skin rubbed, however I have noticed that it seems to be spreading gradually. She is not scratching or showing any signs of irritation and the area is not red or sore, the skin and coat is actually healthy. Does anyone have any suggestions to what the cause might be? At first we thought it may be related to thyroid problems as she is short and plump, but she has no other symptons. Obviously if it starts to get worse we will take her to the vets, but for now i just wondered if anyone had similar problems?

Could be demodectic mange, that usually starts round the eye area this  condition is not usually itchy etc....Vet can do scrapes to test..