Mould at the wall to ceiling joint, particularly in a cool bedroom or a closet, may not indicate water leakage at all from above. It can be a very cold part of the wall simply collecting moisture from the air in the house. This kind of condensation can easily feed mould growth.
One solution is to provide a little insulation over the wood structure that is at the top of all walls. This could be a bit of Styrofoam on the outside up inside the soffit region, or even decorative foam (flat steps of Styrofoam, or crown molding made of foam) on the inside. Insulating the double header at the top of the wall will raise the temperature of the inside surface and reduce the potential for condensation.