There are four fav-icons which get triggered by different states (at least I think you mean this one:));
In order of how it’s decided (first match counts):
- Sync (when at least one folder is syncing)
- Notify (when a device is unknown or a folder is stopped/unknown/outofsync/error or some other notification is open or gui offline, no gui password, device/folder pending, …)
- Paused (when all used devices are paused except (this) one)
- Default (none of the other three states is true)
The exclamation mark would be number 2.
But I have to say that since it’s not inherently clear what triggered the status-icon (clear, as in easily noticeable in the gui), the value of at least the exclamation mark is of a debatable level.