I don't see a timestamp on the notifications, but there may be one. Could this be the next great plugin? Something to automatically clear notifications after a set number of days. The number of days could be set by the admin.
I remember as well, but the only thing I could find was about it not resetting after you viewed it. That was fixed a few updates ago, but they still hang around for a while.
maybe its set by number then cron, it would have to clear itself because if not i would have a kazillion on my oxwall notice list but i think their are only like 50 or so