As far as finding out who the cat's real "owner" is, you're a bit limited unless you can get hold of it and attach a note to its collar. We did this successfully once with an over-friendly and sleek ginger cat who seemed to be on his way to adopting us, but turned out to have an owner who lived in a nearby house who worked odd shifts and had a lodger who was not terribly reliable at feeding the cat in her absences. She left us a very slightly miffed message on our answerphone to say Puss lived there and was lying happily on her bed whilst she left the message. Only when my wife listened to the message later on her return from work the same cat was happily having a snack from our cats' bowls... I am assuming you have done the obvious stuff like asked the neighbours? If you want to discourage it coming in then you're caught between a rock and a hard place if at the same time you don't want to curtail your own cats' freedom of movement. If it's food that's the main attraction could you feed your cats earlier then remove the dish later in the evening? Maybe your cats will in time get used to the intruder? We have had a succession of feline freeloaders over the years through the ever-open catflap, some more brazen than others. (Our own cats are not territorial and so laid back and pacifist even the squirrels laugh at them.) A couple of these characters clearly come and spend the night in the winter sometimes. But even though two of these visitors have been regulars there do seem to be long phases when, for whatever reason, they suspend the visits. Your troublesome visitor may do likewise eventually.