On my upstairs back room there are these brown stains/blotches on the wallpaper. These are on the exterior wall. I've checked the brick work on the outside for the sign of any penetrative damp, but there are no signs. The blotches are mainly located on the top of the wall, as in the image, but there are some more isolated patches further down. I've checked the plaster behind the wallpaper and there appears to be slight staining of the plaster but pretty minor.
Any ideas?