I recently received a 2012 MacBook Pro running Sierra for some development work. I'm quite new to Mac OS X, but was able to figure out how to enable fast user switching in order to lock my workstation when I'm away.
I've noticed however, that when I lock my workstation, a translucent capture of my desktop appears behind the lock screen and if you look hard enough you can see what I was doing before I locked the computer.
For example, if I were to lock my screen this very moment I would be taken to the login window but if I look hard enough I can read the text that I am writing for this post.
Is this a bug or a feature? Has it occurred to anyone else?