I do see a sandbag wall in the database that was built on a standalone piece of land (level 2) yesterday at 5:35pm. This level 2 land isn't part of a base, it's just there on its own.
Perhaps you did it before moving the base and now that the base is here it's no longer possible to see the old land?
I've never experienced buildings appearing out of nowhere before, but I'm happy for you to keep a free wall; it's only going to help your defences!