It should be that a Click on a unread notification leads you direct to the Post, instead only to the Chat, Thread or Forum. Especially the Chat is important. imagine that you get a mention in #general during your absence for an hour and then you put the Computer off. Next day you came online and get the Message "17 Hours ago in #general" you wont spent half the day to scroll in this Chat so this notification should delete if you go offline or you should be able to jump direct on it. maybe a setting point you can choose the behavior.