I ran Windows' tools without getting much back. So I started looking for tools in my "Software" folder. I found TreeSize Free and ran it.
The answer stuck out like a sore thumb. It was in C:\Users\%username%\AppData\Local\Temp. In a future post, I'll explain what had created all those files.
Back to TreeSize Free.
TreeSize Free is a free utility from JAM Software. Its visualizations make the utilization of space very obvious.
It's available as a portable app and you can get it here.