The strange cats owners may put the cat out at night (thinking its the right thing to do but its not) and the cat may be cold, hence coming into yours. I suggest you lock your cat flap overnight, I doubt your cat wants to go out when its freezing at night. in any case. This way the strange cat will learn that your house is not an open house and will move on. I suspect the wailing is to warn your cat that he is coming in or is around as he doesn't want confrontation, merely the warmth of your home!