How can it exist?

My colleague played minecraft singleplayer recently. But after a time like this after half an hour there was fog and strange noises until the message did not come ATTENTION HEROBRINE NEARBY. My colleague quickly teleported far away. How can that be?

How can what be Everything is possible with mods.

There were no mods in the game he showed me

Then it was a server with plugins or a command block that just wrote a message. From nothing, comes nothing.

Can can?