“You have the latest version of WordPress. Future security updates will be applied automatically.”

Since version 3.7 WordPress has automatic background updates.

By default, only minor releases – such as for maintenance and security purposes – and translation file updates are enabled on most sites. In special cases, plugins and themes may be updated.

Of course, as with many things in WP, you can modify that behavior with config or filters.

So to disable these automatic updates, you can:

1. Add this to your wp-config.php file:

   define( 'AUTOMATIC_UPDATER_DISABLED', true );

2. Or disable only core updates (also in wp-config.php):

   define( 'WP_AUTO_UPDATE_CORE', false );

3. Or use one of these filters:

   add_filter( 'automatic_updater_disabled', '__return_true' );
add_filter( 'auto_update_core', '__return_false' );

So check your site for any of these…

You can read more on that subject here: Configuring Automatic Background Updates