Where are my "storage PD capacity" charges coming from?

I'm new to google cloud. I'm managing several users who are testing it out and I run the lab so have to figure out what it's costing us.

I'm trying to understand where the charges come from. Here's current charge estimate:

The bit that confuses me is the "Storage PD capacity" part. I get that this is 'Persistent Disk Storage' but I cannot see which VMs it is associated with. From the charge breakdown I can see that there is 481GB that we are being charged for. But here is the disk space associated with current compute engine instances:

So there is only one instance that has a disk allocated which even could contain 481 GB. But if I ssh into that and check how much disc space is used with df -h then it shows only 38 GB is being used:

So where is the 481 GB coming from? How do I figure this out?

Many thanks for your answers!

The Storage PD capacity is the cost of Persistent disk, attached or not to a VM. It's space reserved for you and thus you need to pay for it, even if you store nothing in it.

Now, why 481? Firstly, London is Europe-west2 region. If you take all the disk in this region, the total is 500 + 100 + 10 = 610Gb.

Your billing report ends on the 25, if you take 610Gb over only 25 days, you get 491Gb (610/31*25), roughly what you see (depend on exactly when the billing has been figured out)