Core

Function writeSearchStat of SearchFormController.php triggers a debug message in FE, because $this->fe_user->id resolves to NULL and cookie is a non-null column.
(possibly the same will happen in SearchController.php)

$insertFields = array(
                        'searchstring' => $this->piVars['sword'],
                        'searchoptions' => serialize(array($this->piVars, $sWArr, $pt)),
                        'feuser_id' => (int)$this->fe_user->user['uid'],
                        // fe_user id, integer
                        'cookie' => $this->fe_user->id,
                        // cookie as set or retrieve. If people has cookies disabled this will vary all the time...
                        'IP' => GeneralUtility::getIndpEnv('REMOTE_ADDR'),
                        // Remote IP address
                        'hits' => (int)$count,
                        // Number of hits on the search.
                        'tstamp' => $GLOBALS['EXEC_TIME']
                );

This seems to have started after 6.2.0beta3