Random blocks

Random blocks keep randomly appearing that I have not been to.  I am building a stadium.  I have been working on one side of it and when looking around it to keep things symmetrical I will find a random block that I didn't place in a spot I hadn't been to in a bit (I know it wasn't an accidental placing because I didn't even have a block in my hand while in that section for this reason to make sure it wasn't me).