Does your cat scratch the inside of his/her ear flap for no apparent reason? When you look at the inside of the ear there is nothing wrong except perhaps a small scratch because he is scratching the inside of his ear!
I think is not that uncommon. What happens is a cat has a slight irritation inside his ear and he pokes and prods it with the claws of his hind leg.
This can break the skin in a minor way. This in turn creates a real reason for a slight irritation inside the ear. Then he scratches again and this time breaking the skin more profoundly and so on until a cycle of ear scratching has been set up.
This is inadvertent, minor self-mutilation causing ichiness in the inside of the ear flap. Obviously sometimes there will be a genuine problem in the ear such as the classic ear mites or an infection of some sort but sometimes the problem is self made.
I think the answer in dealing with this is to make sure the claws of the hind legs are trimmed. This would prevent the claws scratching the skin inside the ear. Hopefully this would break the cycle.
There may not even be a itch in the ear at the outset. It seems to be a habit that some cats develop.