I am having the same problem. I can watch my free space disappear before my eyes. No files seem to be growing (did directory analysis). When i reboot, the disk space is back, but immediately starts disappearing again.
I fixed this once by doing a minimal boot and slowly adding back the start-up apps/services until it started happening again, trying to find the culprit. But i eventually added all my start-up items back, and it stopped happening. So, i fixed it without knowing what the problem was, or really changing anything.
Well, it just started happening again. I haven't found any solutions on the internet, yet.