Just launch app, allow it to receive notification events, select reminder interval and important applications. That's it. If there are any unhandled notifications from thet selected apps in the statusbar, app will notify You periodically with the default notification sound.
The battery is used only if there are missed notifications which requires periodical reminder. App wakes phone to perform sound notification and then sleep again.
The app uses minimum set of required permissions.
Android 4.0-4.2 support is experimental such as these OS versions doesn't have a 100% working method to monitor notification removal.
Source code is available at github: https://github.com/httpdispatch/MissedNotificationsReminder