No, not water damage. It is an outside wall but somewhat protected by a storm porch. However, there is an electricity meter box on the outside and the house supply cable comes from there, through the wall, then to the nearby fusebox and distribution point. I suspect the hole in the wall is not sealed. It is hidden by a large wooden plate on the outside and plasterboard on the inside. I expect if I liberally spray expanding foam around the area it might help. But I shall wait for warmer weather.