So - I found a way to clear this (with a bit of help from this thread: community/community#6874)
Here's the process I used:
- Goto the
Donetab - Click
select allthen clickmove to inbox - Goto the
Inbox - Click
select allthenmark as unread - Click the link that says
select all xxx messages - Click
Done
Hopefully this should clear the ghost "unread" message!