Wow. Holy crap. Wow. Duh!
I didn't think of my cave as an ideal acoustic space but I didn't think that the ambient noise was very loud. It was fine. Until now. Now the quiet is almost oppressive. I can hear my laptop and it's a little annoying, actually.
So the lesson here for recordists is if you don't need it in the room, maybe it should be somewhere else. And if it can be switched off, do it. Your ears will thank you.