Ah,the Midnight Stalker eh? In our last house, with five fairly young cats, we left the cat flap open 24/7. But here, with our two being quite elderly, we lock it during the night. That way, no surprises arrive by moonlight. (Such as half-dead rats, mice, rabbits, shrews etc brought in from the fields as interesting 'toys' for nocturnal amusement by the hunting team!)