Then I moved to the master bedroom. There I started making a slot too! And I noticd the wallpaper just below the ceiling was loose, so I pulled that off. And quite some plastering came along. Oh dear! That's not good. It looked like there had been water ingress. The plastering had small gullies in! And muck had got between plastering and the underlying stuff. Hm!
I think that means the connection between old house and extension leaks a tiny little bit. The extension has been there for decades, and the damage is slight; that means the problem can't be big. But I will need to tackle it! So, yet again, more work to do. Or to pay someone else to do. Oh dear!
The plaster turned over; notice the gullies
The slightly damp top of the wall