Most likely whoever finds them won't have the technical knowhow to do anything with the drives. Given that the filesystem's indices are gone it would be a difficult job to piece much back together. I really doubt that anything is at risk. If you're at all worried, either do this:
cat /dev/zero > file.txt
...wait for it to fill up the entire drive's empty space...
rm file.txt
Or...
install and run this:
http://manpages.ubuntu.com/manpages/...erofree.8.html
Both basically overwrite the free space...
I personally think both are overkill if whatever you had on the drive wasn't all that important and you've already installed Linux over the Windows partition.