There was a MOUSE nest in my yarn.
Which is ruined. All other yarn is down where it can be seen and examined. (So far, everything else is fine.) I've had it with rodents. I am traumatized. STEPS WILL BE TAKEN. Before any more yarn is lost!!
(Seriously, we'd gotten all the little buggers out of the house - it's probably a really, really old nest in the one yarn bag I didn't zip properly. But STILL. TRAUMA.)