Spawn of Satan
We went away for the evening, staying over with friends. We've done this a few times. In early instances we locked Binks in with a litter tray but as he got older we just made sure there was some food down for him and pledged to be back a good time the next day. This would be about the third time we had left him for the night in London.
When we got home we found quite a scene. Binks didn't greet us at the door and on checking upstairs I found evidence of 'territorial marking' on the stairs - I'll leave it at that - and a strange black cat asleep in our room on the bean bag Binks normally uses. The foreigner seemed more-or-less unperturbed and I closed the door as our cat came down from the floor above - emboldened by hearing my voice no doubt. We stowed him away and 'dealt' with the intruder. That amounted to try to grab the scruff of his scrawny neck, failing and then chasing him out, downstairs and through the cat-flap.
We had to clear up the stairs and wash the bean bag cover, complete with a deluge of min-polystyrene balls as it had burst last time we did this (years ago). Binks was jumpy for the first hour or so and keeps looking towards the door. That was three or four days ago and he's now back to his relaxed self and there's been no sign of Satan's spawn.