I have an inside, interior wall that is plaster. There is a long rectangular dark "shadow" about 18" wide going from the ceiling down about 4'. No roof leaks that we are aware of. It doesn't feel damp. Looks like a shadow but is there all the time. The house is gas heated, built in 1984, located in Ontario. There is unheated attic space behind the wall, no access. Does anyone know what this can be? I can only think it has to be something bad.