At first I thought it was time machine so I did
sudo tmutil disablelocal
But that didn't fix it. Then googling showed it could be a swap leak, but as I said, DIX didn't show any of those, and it does show swap files (It currently shows one as I write this)
And then I tried booting into safe mode, single user mode, yet it didn't work. It got stuck halfway.
Then I did a verbose boot and I got
AppleUSBMultiTouchDriver - device was reinitialized (something alone that line)
And I said **** it, deleted some steam games using terminal from a 10.9 recovery USB I had and installed 10.9 over 10.10. Now I seem to be staying around 15GB free, but I should have around 30GB more that I can't find specifically.
I suspect the culprit in all this was the Razer Synapse 2.0 driver for my gaming mouse. I belived it to the be the issue when I first updated to the 10.9 beta when that came out, and eventually got it fixed without wiping. I'm still not sure though why that would have been filling up my HDD either. So I suppose I either have to uninstall it, or stay on 10.9. I'm going to try uninstalling it soon then reupdating. We'll see how that goes.