Hello dear, Welcome to HCM. I have read your query and understood your concern. Such bald patch behind ear is most likely due to a condition known as alopecia aerata. It is an autoimmune condition of skin characterised by patches of hair loss. It is a self limiting condition. But in case , the patch increases or new patches appear, treatment is advisable. But before that, diagnosis should be confirmed. I will advice you to visit your dermatologist in person or upload a pic here so that diagnosis is made. I hope it helps. Good luck.