Worlds not unloaded

I opened up a world in which I had a contraption involving a minecart with a hopper. I closed it and opened a new one. As I moved through the new world, I heard the minecart from the old one around the coordinates where it existed in the first world I had closed. Sounds like the old world was being kept track of by Minecraft. Same with chickens and other things.

 

This can easily be tested with Superflat.