I heard a story once that a cat with one of those microchip cat flaps was actually standing there holding it open to invite all his mates in for dinner!
Can you get near the mystery cat? If so, cut a inch wide strip of paper, write your phone number on it along with the words "will the owner of this cat please call *** ****" and lightly sellotape it round the cat's neck like a collar. If it has a caring owner they would want to know what their cat is up to.
I'll have a fiver on it being a male because female flap raiders are like rocking horse poo. They do exist I am sure but I can't remember dealing with one.