Damp mystery. Where is it coming from?

Have a small cloakroom. Solid walls, no cavity. External window wall is about 12 feet X 12 Feet. Radiator under window. Was going to redecorate and found under window and at both sides seriously damp with paper peeling off. Behind the radiator, under the window, actually was a piece of plasterboard. Very damp. The plaster was boast up to about four feet. Maybe the term is blown. I chipped it off expecting to find a soaking wet wall. Wall is bone dry everywhere along under the window and up both sides. Where could the damp have come from. Can it be condensation behind a working radiator? There is the gas boiler on one side of the wall but I have checked all round it and even the condensation pipe through the wall and all ok and dry. Behind the radiator are the heat reflecting silver panels. Can anyone shed any light on this puzzle?