Not sure how visible it is on these pics, but I was in the loft a couple of days ago after SWMBO was up there and encountered a couple of wasps. When she disappeared, and hence silence ensued, I couldn't hear any tell-tale b uzzing of wasps (the two she had encountered had been dealt with), so took a look around our cluttered loft to see if I could see anything untoward. There's no real reason why a couple of wasps couldn't have gained access wi thout there being a nest in there.
Anyway, I spotted unexpected trails of whitish dust - they looked like some sort of calcium type deposits, and the trail of them was solely underneath the valley where two roof sections join at right angles. I've not noticed this before, so I am assuming that this is a new development in an otherwi se relatively unused area of our house. Photos arent great, but hopefully you get the picture.