Hard drive suddenly full - Not Log File or usual suspects

I have suddenly got two Lion computers that in a one hour space filled up the 20GB free space on our hard drives, and of course is constantly flashing message HDD is full, duh.

I have been using a 125GB SSD drive for my Lion OS and Apps, which would be about 100GB. I have been keeping that last 20GB free. I had to manually delete downloads and other folders, and move music to external drive. I keep all data on external drives, but it has been working fiine for the past year.

Today, I had only Final Cut Pro open, exporiting a MOV file to an external drive.

I went to the store and came back, and Java 7 came up and was prompting me for the new Java 7 Build 15 update, and behind that FCP said "General Error" and never exported the file. I closed FCP, clicked to to the Java update, and it gave a weird error and kicked out; and then Lion started freaking out and telling me my hard drive was full, duh.

I tried using Omnisweeper, and checking for a runaway log file, nothing. I cannot find a 15GB file, and I have no idea what is going on. Is there a update downloads folder for eitehr Mac OS X or Java I need to delete? Can anyone help? Any suggestions?

Thanks

Craig G

DC