You should make a backup of your data immediately -- the drive is failing. Bad sectors mean the drive needs to be replaced -- by the time you see one, all the spares have been exhausted (every drive is built with spare sectors that are silently called into use by the drive's logic board).  Once you see a bad sector, the damage  has propagated.  
As per the suggestion of ejn63, I ran the ePSA diagnostic.  No RTC issues but did get the "dreaded" 2000:0142.  Fail PSA V4213 Status = 7 So, should I panic and look for a replacement drive immediately or could it just be a bad sector or something?  I still have no apparent symptoms.. Boots fine, runs OK.  No real delay in accessing data or programs generally. Thanks.
Results of ePSA