I had been running L4 (4.5b) for a few days with some visible sync errors and having to click on the icon, but now the Sync icon has disappeared (blank).
1452.SyncError.zip
EDIT: I had just deleted a few Syntax items in beta 4.5c. When I restarted 4.5b the sync icon appeared, showed the Warning triangle and then resumed sync'ing. The Syntax items were removed as per 4.5c.