Why is there a red dot on my iMac system preferences icon?

The System Preferences icon on my iMac dock has a red dot with the number 1 in it. However, nothing in the System Preferences window itself or in the Notifications Center list shows any indication that a notification is 'active'.

Reading elsewhere, the red dot on the Notifications icon seems to be part of the icon design, but why the red dot and number on my dock icon?