1TB Seagate Barracuda ST31000528AS - Unexpected "Offline uncorrectable sectors"

1. Yes, it definitely looks like a hardware fault.
2. I have had multiple Barracuda ES / ES.2 drives go..including 12 out of 16 across two servers.
3. I don't think the smartmontools is sufficient for warranty replacement. Go to Seagate's website and download their SeaTools bootable CD. When/if it determines the drive has a fault, it will give you a warranty code.

To me, it looks like you have a dying drive, with sectors that can't be re-mapped. Ditch the drive, get a new one, rebuild the RAID array. The drives are so cheap it's really not worth thinking about. Especially if you have any attachment to any of this data.

edit: Additionally, SeaTools will give you a list of the bad sectors that it finds. Also, output of smartctl -a /dev/sdb would be helpful.

edit2: From looking at your smartctl -a output...

You have 2 "Offline Unrecoverable Sectors" --but what does this mean?

An offline uncorrectable sector is a disk sector which was not readable during
an off-line scan or a self-test. This is important to know, because if you have
data stored in this disk sector, and you need to read it, the read will fail.
Please see the previous '-C' option for more details.

citation: http://smartmontools.sourceforge.net/man/smartd.8.html

There are also 2 "Current Pending Sectors" --and what are they?

A pending sector is a disk sector (containing 512 bytes of your data) which the
device would like to mark as ``bad" and reallocate. Typically this is because
your computer tried to read that sector, and the read failed because the data on
it has been corrupted and has inconsistent Error Checking and Correction (ECC)
codes. This is important to know, because it means that there is some unreadable
data on the disk. The problem of figuring out what file this data belongs to is
operating system and file system specific. You can typically force the sector to
reallocate by writing to it (translation: make the device substitute a spare
good sector for the bad one) but at the price of losing the 512 bytes of data
stored there.

So, your disk was unable to read from two sectors, could not, and now wants to mark them as faulty so they can be reallocated.

Personally, I tend to toss drives (especially in RAID arrays) once they start having bad sectors. But, I also don't pay for them...

Alternatively, maybe you could break the mirror, remove that drive, and give it a scan with SeaTools? If it is able to remap the sectors, it will. It shouldn't matter if/when it destroys, because the mirror will be rebuilt when you re-insert the disk and rebuild the array. However, I've not done this, and would wait for someone to chime in.