I’ve just updated an installed Drupal instance to version 6.15. On checking the installation I get this:
“Drupal core update status Not secure! (version 6.16 available)
There is a security update available for your version of Drupal. To ensure the security of your server, you should update immediately! See the available updates page for more information.”
You can set up your system to download new install scripts immediately when they are released – I belive 6.16 Drupal has been out as an install script for a while now. Go to System Settings->Script Installers, and click on the Installer Updates tab, which will let you download scripts immediately rather than wait for the next Virtualmin release.