Windows 8 - Odd HDD behaviour

No idea what the hell is happening here.

I have Windows 8.0, on an Asus laptop with a 1TB HDD. I have several partitions.

1 for OS
1 for Games
1 for Apps
1 for Java Dev

On the OS drive, I have allocated 50GB, 20GB of that is taken by Windows.
I also have 9GB in the User, just random crap I intend to delete
I have 4GB in program files, some software is shit and you can’t tell it where to go.

So that is 33GB. I should have roughly 15GB left, I have 200mb and I can not figure out where the hell the rest has went.

Yesterday, I had 0 left, I deleted 15GB worth of crap from Temp files, I checked today and Temp files is only 3mb.

Any ideas to where this 15GB has vanished?..