This happened to me a few years ago, at the time I was playing Minecraft on an Xbox 360 since it was the only way to play it. For the timeline this was before the banner update.
Well when playing I had decided to create a new world since I had gotten bored in my other world, so I created a creative world. When loading in I had spawned in a hilly biome if I remember correctly.
Well I looked around a little, thinking if I would build there but had decided against it and decided to just fly around.
Everything seemed perfectly fine, I would stop flying for a few moments randomly just to get a good look around or grab something out of my inventory.
Everything was going fine it was like any other Minecraft world I played… At least at first it was.
At some point when I stopped flying, I didn’t look around I was just floating there. I don’t know why, I think I was possibly talking to my older brother at that moment because he was watching me play.
Well I ended up noticing on the side of the screen, and when I looked. I saw a “player” I was greatly confused by how someone else was in my world.
It was a single player world, and I couldn’t even make a multiplayer world if I wanted to since the Xbox wasn’t connected to the Internet.
My parents didn’t want us to do something on it that we shouldn’t, so anytime we played any game we were always alone.
And I even checked something, I don’t know what it’s called but it showed the list of players who were in the world. I was the only one there, and there was no message that someone had even joined.
Well even after I left that player list screen the “player” was still there, just standing on a hill and watching.
Their player skin was black but it was also kinda see through, it was weird.
Well I ended up flying over, curiosity out shining the fear I felt. But… When I went where it was, it was gone.
I don’t know where it went, I had looked around. Thinking that maybe that it was still nearby, but no. It was completely gone, and it wasn’t possible that it was blocks that looked like a player somehow.
And it couldn’t have been any mods since none of us (my brother or parents) knew how to even get mods onto the Xbox, and my parents don’t even play the Xbox anyway.
I just assumed that the “player” left or something, and continued flying around.
That wasn’t the end of it though, I kept seeing it around. It was still watching me and whenever I went near it just disappeared, and it was honestly terrifying.
Never once had something like that happen in my other worlds, well I got too freaked out and ended up giving up the rest of my turn on the Xbox.
I left the world and never got on it again, and even when making new worlds or playing old ones nothing like that happened again.