Okay, magic has rules, it follows laws of physics. What we can do with it is based on a lot of factors involving those laws - I can track someone with a piece of their hair or the blood from their relatives if there's a strong enough bond. But tracking the hair taken from someone is a reverse order spell, so to speak, and while possible if I worked it right, would be more beneficial if I had a piece of the person who took it to begin with.