Ok guys,
lets say that Rd does exist as a fair hypothesis, and that it is rare, but not unknown, (leaving aside the possibility of leaking pipes, as, if the leak is stopped then the problem is presumably solved).
However condensation in modern(ised) houses does exsist, and I would say in most, due to the lack of unplanned ventilation, (drafts)
When I were a lad.... we only had single glazed windows, which had beautiful ice crystals on them inside every cold winters morning, (and that means Nov to April).
So, every human exhales about 2 litres of liquid in their breath every 24 hours, where does all that water go?
It 'finds' the coldest area in the house, typically behind a wardrobe on an external wall, but also can be any cold corner in any room ,also kitchens bathrooms etc.
Think about it....