Are you absolutely certain files were downloaded? Were you monitoring your Net traffic?
Maybe the OS got confused and your files accidentally got the "in use" tag. This might happen when you're doing complex stuff (developing ir debugging video games).
Does your computer have a LAST ACCESSED date?
TO PREVENT:
Get a fingerprint reader. You can store the files in your "safe" accessible by a complex password or simply your fingerprint. Try this:
Microsoft Mouse and Keyboard Hardware – Fingerprint Reader and
Compare Prices and Read Reviews on Fingerprint Reader Scanners at Epinions.com.
Good luck! Hindsight is worthless.