Maybe it's going to a system restore folder or something? Vista has a habit of doing that.
Go to Folder Options and enable 'Show hidden files and folders' - then do the highlight+properties thing.
Incidentally if you're worried about space, install+run CCleaner.
That's with all my hidden files showing, unless it skips certain files. I wouldnt be worried if it was a few hundred megs, but this is 54gb that Im missing
EDIT Lock it up. Turns out I had 52.1GB in offline folders...