I use a mid-13 13-inches Retina MBP 256gb with macOS Sierra 10.12.2.
I recently decided to securely erase free space using the following terminal command:
diskutil secureErase freespace 1 /Volumes/HD
After completing its task, I got 50GB of free space instead of the 80GB I had before the operation.
Could it be a remaining temporary file used by secureErase?