Strange SID appearing under user security

Recently I have been going through a class that teaches how to operate windows 7 , and we covered permissions. I decided to try it out on my own computer and I saw a random SID with a red ? on the person, that was not apart of my user list. It then suddenly dissapeared like a cocaroach from the light, and it has me concerned about my security. New Picture.jpg Someone help. None of my instructors knew what it was, and they are heavily certified professionals...