If it's a 1 in 10,000 chance, and if Minecraft has been out for 3 years (that's quite a generous number) you'd have had to boot up Minecraft almost 11 times a day in order to have likely seen it by now.
I'm it's overlooked more often than not. Notch probably meant to keep us off balance thinking it was a hidden feature (aka, "removed herobrine.")
Jesus christ HOW MANY PEOPLE THINK HE EVER existed?!