Just noticed this brown stain above my living room window. I don’t have a ladder at the moment so can’t reach it to touch it but the wallpaper near it is starting to lift so I think it might be damp. But it’s on the ground floor and there doesn’t appear to be anything else around it or in the room above it. My bathroom is on the floor above too but to the right. Could it be water travelling from the bathroom down or getting in from outside? It hasn’t rained locally since Saturday.
As others have said, 100% a leak.
I would advise to stand on a chair or stepladder underneath the ceiling and check to see if it is still level. If you see an obvious deformation around the stain, this will be being caused by water pooling on top of the ceiling plasterboard. In which case, once the leak is sorted, you will likely need to drain the pooled water, cut out the damaged section, replace it, then replaster and repaint.
We had exactly the same issue in our last house. It was in a difficult to see spot hidden behind our kitchen cabinets. We only realised the severity of the issue when the ceiling boards gave way and fell on my head.