The earth is flat like a coin. Some of the dead live on the underside as ghosts "Hell". Tortured by the exposure to constant UA, and the darkness of space (Sun is top side), a few occasionally make it to the sunward surface and roam about in dusty old houses. On Halloween, great armies of them begin a trip starting at dawn, crawling over the rim, past the ice wall. By Midnight (The witching hour), they have made it to the northern Hemisphere, where they spook the good citzens of the living world.
Now I suspect you were hoping for a silly answer like this, but nowhere does FE have anything in it about ghosts. Just because FErs suspect the earth to be flat, does not mean they suspect the existence of ghosts, big-foot, unicorns, leprechauns or the Easter Bunny. The very point of Zetetism is that you observe and test something to make absolutely sure that whatever you observe, is and can only be identified correctly.