Maintenance mode says it's on, but site is still available Before attempting to do some module updates, I enabled maintenance mode on the site. I then noticed that the site was still available to all visitors, even though the Configuration->Development->Maintenance Mode page said that maintenance mode was enabled.

I have cleared all caches (via admin pages and drush) several times already, and have checked the settings.php file to see if there was a manual override set. I have also turned the setting on/off several times, logged out and logged back in, etc. 

I've never seen this happen before. What else can I try?
Have you checked the permission "use the site in maintenance mode" to see whether it is enabled for other users?