Im no expert on cake php - but a simple install + test + documentation at ..
http://book.cakephp.org/view/1584/Error-logging suggests that when your debug is turned to 0, your logging still continues logging for warnings and fatal errors therefore to turn of debug completely you also may need the logging turned to false.
Configure::write('log', false);
Extract: Errors are now logged when Configure::write('debug', 0); Would log only warning and fatal errors. Setting Configure::write('log', false); will disable error logging when debug = 0.
Also in the base controller.php (at libs/controller/controller.php there is no handleError code - was this added by your developers?)