check system restore, see how much space allocated to it (probably on C anyway but worth a look).
run defrag (even though they say not to on ssd, it won't hurt - win 10 runs it once a month anyway)
could it be steam updates?
is hdd activity normal? Normal should be 0, any processes constantly using space?
Try booting into a linux live USB and see if it shows the contents of the drive. I don't get how its not showing in windirstat. There is a huge discrepancy between front page showing size of drive and space used, and what actually shows in the directory structure. Something is missing
not a virus
not maleware
not ssd - unless 2 tests are lying
not torrent (Did you delete it before and remove folders?)
not page file (as it should show in windirstat, I assume its on C partition)
running out of ideas besides a format of the partition. If that doesn't fix, need to look at what programs you are running.