I was in single player mode and I swear I saw another player. He had the default skin but his eyes had no pupils. I double checked I was in single player and i was.... I also started finding things that looked like they were built by a player. And I never been there before. And this isn't apart of the update where you can find villages. So Yeah. 

Mod Edit: Stop making things up, Herobrine is not real. Also, Notch Doesn't have a brother, he has a half brother he never met.