It was a bug. The vet put Rock on antibiotics and he's already feeling better and eating again. He was curled up in his basket inside a dark closet for days, a sure sign that something was amiss. He does seek out his basket for comfort, the way a child will suck his thumb, but there was no doubt that he was under the weather, and in his delicate state of health a bug can be life-threatening.
This bug is a young Brown Recluse spider (Loxosceles reclusa) resting on the side of an umbrella stand in the breezeway on a very cold night. He looks fit, and that's exactly how he ought to look going into winter, but I haven't seen him since. He may have frozen to death, but chances are good that he crawled beneath the breezeway floor and found a cavern of warmth somewhere.
I like spiders and make it a point to leave them be. Still, I hope the bug that laid Rock low does the same vanishing act – for good.