You may have clicked the “Mark ALL messages as read” button. There is a clue in the button label.
I#ve never even seen that button before... had to go looking for it after reading this post.
Suppose I could have clicked it by accident but don't think so.
I'm not arsed (though there were some threads I was reading through and lost track of where I had got to). Was just curious if something had happened and had affected everyone.