[I'm presently teaching my dog English but it's taking her quite awhile to get the hang of it. She's always complaining that I told her that after fractions and trigonometry, English would be a breeze.]
I don't use this form all that much in my active language but I too know the meaning and that it's okay when I see it used.
I'd say that it's used in more formal instances such as police reports;
Person or persons unknown