Since I wasn't here when this happened:
What is your propane furnace connected to for venting?
I have a double flue chimney.
The propane furnace is a stainless liner in a separate flue.
Did the propane furnace run on that morning the CO alarms sounded?
It may, possible, have run sometime over night while I was asleep but not within 6 hours or more of this event.
Can you rule out the possibility that the propane furnace could have caused the CO problem?
With near certainty.
The coal furnace has been keeping the dining room in the mid to high 70s once I get it cooking for the day.
The propane furnace at 65.
This is the same room that has the propane thermostat and the CO detector.
The detector is sitting in the middle of my dining room table.
I'm not buying that exhaust leaked from the furnace and nobody smelled burning tires in the house. Whenever I had even a mild puffback with bit coal the whole house would reek of burning tires
As I said, neither my wife nor daughter have situational awareness.
It may have smelled of burning tires and they may not have paid any attention and they can't remember anything about what they were doing or what was going on other than the alarm went off.
I'm tickled if it's not the coal furnace.
I do think Paul's theory of two separate detectors reacting the same way and giving the same "false" reading sitting within a couple feet of each other may be a bit of a stretch.
But, it's as good as any other idea right now because neither detector has moved from -0- since relighting last evening.