What is the tile under the mouse pointer in the screen shot? (Just above the vending machine.)
Even a vacuum would have a tool tip, but this tile does not. There is no tooltip for the tile and you can not click on it.
It may be similiar to the fog of war, especially seeing how it is in a POI of the vending machine. But a fog of war should have been removed... I rummaged through the vending machine already, and I deconstructed the security door. (A few tiles to the left of the vending machine, the copper is still on the ground.)
load save game, examine tile.