While this “Ghost Rave”–as I’m now calling it–is nevertheless awesome, I think it has to do something with the lighting code. This happened with a saved game that was already paused when it was loaded; unpausing seems to exorcise the ghosts.
The only sources of light I am using at the moment are the Fire Pit and Wall-Mounted Wooden Lanterns, so I assume the glitch might be originating from one of these.