The disk that shows up on AWS EC2 instance – where is it coming from (among the AWS EBS volumes)

I have a running AWS EC2 instance (Ubuntu 12.04.4 LTS). When I ssh into that instance and do a 'df -h', I see the following:

Filesystem      Size  Used Avail Use% Mounted on
/dev/xvda1       99G   75G   19G  80% /
udev            1.9G  8.0K  1.9G   1% /dev
tmpfs           378M  172K  378M   1% /run
none            5.0M     0  5.0M   0% /run/lock
none            1.9G     0  1.9G   0% /run/shm
/dev/xvda2      394G  199M  374G   1% /mnt

Please note the last line in the above output, which is

/dev/xvda2      394G  199M  374G   1% /mnt

Now here is my confusion. It shows 394G of disk that is mounted in the directory /mnt. But in my effort in trying to figure out where this volume is coming from, I seem to be going no where. In order to find out where this volume is coming from, I logged into my AWS EC2 Console and then looked at the list of attached volumes that I have in there. I assume the above volume is attached, given it shows up on an EC2 instance. But when I look at the list of attached volumes, nowhere do I see a volume of around such a big size i.e. 394G. All I see is something like the following:

vol-<volumeid> 8 GiB ...
vol-<volumeid> 8 GiB ...
vol-<volumeid> 8 GiB ...
vol-<volumeid> 100 GiB ...
vol-<volumeid> 10 GiB ...
vol-<volumeid> 10 GiB ...
vol-<volumeid> 10 GiB ...

None of these volumes add up to approximately 400 GiB

Also when I look at the EC2 instance itself, I see the following disks mounted on this instance:

Root device type ebs
Termination protection False
Root device /dev/sda1
Lifecycle normal
Block devices /dev/sda1

All I see is two /dev/sda1 entries (how come?) and none like /dev/xvda2.

So how do I now identify where the /dev/xvda2 is coming from, identify it on the AWS Console and then figure out how to delete it or manipulate it etc. Hope my question is making sense. If not, please let me know what additional information I need to provide to solve this.

Thanks.