No washer/dryer running, nothing cooking, the furnace quiet, I finally decided it was a low battery/false alarm.
Climbing up isn't much in my bag of tricks any more, but I did get the unit unscrewed from the wall. However, instead of a battery, I saw a plethora of wires.
I called MS, who, laughing, told me to change the battery. When I explained that I'd had that one replaced by the electrician, who'd tied the units together, he came on down, UNPLUGGED THE WIRES, disassembled the unit, where he found a 9 volt battery.
Yup, I got "the look".
Fortunately, I had that size in stock.
He popped out the spent battery, replaced it, then reassembled the unit, hooking it back into the wires, and mounting it on its wall base, so quiet again reigned.