I use Yosemite diskimage encryption on a USB for a few files and the files were 256 aes disk encrypted using disk image encryption "How to create a password-protected (encrypted) disk image - Apple Support" The image was unencrypted and placed in my garbadge. I had created the encryption disk image on my USB drive and it was some how decrypted. The symptoms were my Yosemite Mac flashed a message that I am low on Disk space which is not correct then I noticed the trash can went from empty to filled. When I check the files I noticed they came from my USB and were unencrypted. The USB was connected.