Turns out it's because I didn't scroll up and load the previous 8 days worth of messages and read them all.
Not because there were new messages.
tl;dr mark all as read fixed it.
if you're on desktop, there's a "x" to dismiss it 😉
@robbieaverill has left the channel
@zanderwar commented on @andante’s file https://silverstripe-users.slack.com/files/U2WP9BGS0/F52DPSLKT/pasted_image_at_2017_04_22_09_25_am.png: I have the same mirror in brown, p.s love the walls
@Firesphere archived the channel