The unexplained release of a large vapor cloud on Friday from an Exxon Mobil Corp refinery near Los Angeles prompted authorities to sound public safety sirens, but no toxic fumes were detected and no one was hurt, a fire official said. The mishap was reported around sunset in Torrance, California, at a refinery that had been largely shut down since an explosion and fire at a processing unit on Feb. 18 injured four workers and destroyed emissions-controlling equipment.