BulletOwner1 was correct:
that was the FW [valveless] drain. After hunting all day on Saturday, I ended up paying to have somebody come winterize it for me on Sunday. And boy howdy, I'm glad I did; our high today is 18°F. We're having bitterly cold weather on par with what we had last winter. And it's not even winter!
hankaye, thanks very much for your help! The shower, galley, and bathroom sink are all road side; the toilet is curbside.
So now that I've paid to learn where that dratted FW drain is, I think I'm ready to winterize next year.