Remember this? I think the mystery is solved. Turns out it wasn't a fluke and has everything to do with our heating and cooling systems in the house.
A routine furnace check up yesterday turned into an urgent meeting this morning about replacing the furnace, water heater, and (oh yes) air conditioner. Too many details surround why we need to do all three all right now, but suffice it to say this is a good thing for our house. A bit of a hit for the pocketbook, but darn good for air quality in the Little Cottage.
Oh, to think I was actually starting to dream about remodeling the bathrooms...ha!