Here's what I'd try:
Get an external drive.
Download SuperDuper from here (it's FREE):
download <- just click there
Use SuperDuper to create a cloned backup of your internal drive. It's easy.
You want to see if you can "find" the 100gig of files on the backup.
There are things you can do with a backup that you can't easily do from the internal drive.
Do this with the external backup before you start "hunting":
a. mount the drive on the desktop
b. click ONE TIME on the drive to select it
c. bring up the get info box (command-i)
d. at the bottom of get info, click the lock and enter your password
e. put a check into "ignore ownership on this volume" (sharing and permissions)
f. close get info
Now you can gain access into some areas (such as other accounts) that you couldn't "get to" before.
No promises, but it might help in exploring to see "where those files went".
And one final thing:
Don't you think it's time to learn something about "backing up" your stuff?