How do you know a mystery cat exists?
The food gets eaten, the water is drunk, the litter box is used. But sometimes... the litter box is not used. I think we've sorted it out now but for a while the bed became an alternate litter box.
This does happen occasionally, new cats telling me there is something wrong in their world, and it's annoying but not a big deal. However, when a cat drinks about 1.5 L daily it becomes a bit of a bigger deal.
I placed another litter box in the room and for almost a week I slept on the couch while my bed was covered by 4 garbage bags taped together.
Then I slept in my bed but covered it up with the bag blanket for a few more days. Friday night I bit the bullet and put the bag blanket away. Fingers crossed we'll be ok.