Pressure test is the best way to determine source of leaks - they cost about $100. Not familiar with L your referring to - might be someone placed it there to mitigate known water problem - if so lousy fix. Check the roof caulking everywhere the actual source of the leak may not be in the vicinity of the wet carpet (water maybe entering from different location and traveling along the interior of the ceiling cavity). Water leaks are PIA and if not fixed quick can result in expensive repairs - don't ignore.