If you happen to use Google+ occasionally like I do, it may not be obvious immediately how you access notifications that you have previously read on the site.
As you may know, Google+ displays notifications when you click on the notification icon in the top bar on the website. When you click on the icon, all notifications that you have not clicked on yet are displayed here, regardless of age.
Once you click on a notification, it is hidden from the listing and there is no obvious option to bring it back.