Mysterious hard disk space vanishing?
Might I suggest using one of the myriad apps available at your disposal to hunt down and find the rogue files that plague you? An excellent option that I found (and free to display the content, though not to delete from within the app) would be the DaisyDisk app. It'll show you exactly what's using up your space in a relatively intuitive/futuristic display.

From there, it's a little matter of simply deleting the files in question.