How did COVID show up in the bathroom of a long-vacant apartment, and what does it mean?

The discovery of the coronavirus in the bathroom of a vacant apartment in southern China is being taken as evidence that the pathogen spread up through sewage pipes after a toilet was flushed in another unit. The finding is adding to concerns over the virus' ability to spread in tiny airborne particles in enclosed spaces, and not just in the same room, or even on the same floor.