Im looking but cannot see it..

If there are topics in the list when you click 'Show unread posts since your last visit', there will be a blue button that reads 'show all unread topics'.

If there are no topics in the list when you click 'show unread posts since your last visit', there will be a message stating 'No unread topics found since your last visit. Click here to try all unread topics.'

At least that's what I can see. I haven't had a chance to investigate further sorry.