In OS X 10.8.3 and later, Apple seems to have changed the behavior of the Mac App Store: It will no longer automatically check for System Updates if you are running from an account that does not have administrator privileges.
A simple work-around: Select the Updates tab in the Mac App Store. From the Store menu, select Reload Page (Command-R). The app will now prompt you for an administrator’s credentials. Then the app will search and (in my experience) find system updates if any are available. Updates can now also be installed from your non-admin account