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?