When you have more than one drive in a Raid1 and they are syncing, can be a few reasons why, most obvious is replacing a drive, the sync progress will be displayed.
If you happen to have a system that was setup with just one drive and you install a second drive, the console Manage Disk Redundancy will offer you the opportunity to add the new drive to a Raid 1 and will begin to sync the drives..
You say you are informed in the console menu you have a 1 disk raid, I presume you originally had a at least two..
Added: sometimes a disk will be kicked out of the Raid, it may require manual intervention, sometimes on a reboot you will be offered the chance to add it back from the console menu.