I recently discovered that when maintenance mode has been activated, there is no way to deactivate it, even by an authenticated administrator.
This is because the maintenance mode check is initiated in wp-settings.php, which is loaded directly by wp-config.php (so it applies to ALL requests, including admin, ajax and cron).
It would be useful to have a way for administrators to cancel maintenance mode without having to SFTP in to the site and remove the .maintenance file.
This would save a lot of time for those of use who manage many sites, especially from a central control panel like MainWP. If a MainWP plugin update times out for some reason, we have no option but to remove the .maintenance file before MainWP can reconnect to the site.