But I finally found out what "the kitchen poltergeist" was. I'd been thinking mouse, but while the events (light cardboard boxes falling over, Jiro being very interested in something under the Hoosier cabinet, etc.) matched a small critter, I didn't smell rodents, nor find rodent droppings. That's because it wasn't a rodent.
Three days ago, the mystery guest moved to the living room, and has been hiding in the vicinity of the couch, based on Jiro's interest. Yesterday, I came home, and he'd caught it. "It" was a rather large alligator lizard. Apparently, he's been living off the bugs in the house, and had run short of spiders under the Hoosier cabinet or whatever. Jiro would drag him out to the center of the carpet, let him go, and then pounce on him as he got near to making it back into cover. This is probably how the lizard got loose in the first place. And not wanting him still roving the house, I caught it in a paper bag and released him outside. I don't know what shape it's in, but at least it's not in the house, any more.