So a little more digging lead me to think it was the local backups so I ran
sudo tmutil disablelocal
which removed the MobileBackups folder. Popped open "About this mac" to look at the storage section" and...
It's still showing only 90 gbs free (with my Finder window showing 209).
And here's the REALLY crazy part, the portion that used to be taken up by "mobile backups" is now filled be the "other" block.
Now I'm completely confused.
Just a note that this is happening again on a new machine.
About a week ago I watched my disk space fluctuate from ~715 GB up to a whopping 775+ GB. I knew that COULDN'T be right but a week later it dropped to 693 GB.
Obviously I still have tonnes of space on my machine but I like to KEEP it that way, so I'm pretty anal about this sort of stuff. I'm just confused as to how/why my space would be fluctuating so much. Again, I'm presuming it's local backups or something since I know for a fact I didn't add over 75 GB of material to my machine in the last week (I've barely even used it). So what the heck is going on here?