Guest user randomly created on my Mac, after setting up Find My Mac
With Find My Mac enabled, someone who found the computer could sign into the guest user, which could then be located via iCloud. The guest user is a feature of the operating system which allows people to temporarily use your computer but not save any files.

You can go into Users&amp;Groups System Preferences and disable it, for more details see this article 

Just make sure you don't sign into it and expect files you save to still be there after restarting the computer :)