I would never detract from the advice given by carbon monoxide sensor manufacturers or Fire Service Professionals when a CO detector sounds in your home. Call the Fire Dept, open windows, be alert to symptoms of carbon monoxide poisoning etc.
But is there more to the story? It does occur that an exact cause is not detected since the event has passed by the time the Fire Department attends the scene. Often, Fire Professionals will advise to have household heating systems checked for malfunction. For example; Do chimneys need sweeping?
To this I would add the following:
Do you have gas or oil appliances in the home? Where they running at all?
#1- How old are your sensors?
Do you have 'plug in' sensors? if so, switch the position of your sensors (if one only going off)
If both are going off - then explore other factors. (if only one sensor is activating- you can start by replacing that sensor)
Other questions to answer:
Where you using a clothes dryer or any type of fan that puts air from the inside of the house to the outside of the house when this occurred? - because appliances like these cause depressurization which can pull carbon monoxide out of other appliances, especially those dependant upon natural draft; for this reason I always advise cracking a window in the room where the clothes dryer is located and being used.
It is not surprising to find out that most alarms are triggered between 11:00 and 5:00am when, during the winter months windows are sealed shut and doors remain closed the longest (a door opening provides a fresh air supply to the dwelling).
Not all homes have HRV's (Heat Recovery Ventilation) devices to bring in fresh air from outside and expel stale moist air from the inside of the home. Even, if there is a functioning HRV; when was the last time the filter was checked for blockage?
As far as wood burning appliances; you may need a chimney sweep (we do not know; this being said you should know the condition of your chimney and have a maintenance schedule in place) but, I would not assume that first off if there is no smoke spillage from the appliance.
Now, if this only happened when the stove was down to coals; there is little smoke at that time - so this would not be conclusive. Then again, if the stove is situated in a negative pressure zone (like a basement) or the chimney goes through the wall and up the side of the house; then, the chimney exposed to cold will naturally have a weaker draft that at some point will reverse as the fire cools down. Likewise, a basement installed wood stove will be competing for combustion air with the upper portion of the home and could spill smoke or carbon monoxide when the draft becomes weak as the stove is cooling off.
A very strong wind at just the right angle on the chimney top can also produce smoke spillage from a wood burning appliance especially when the draft is low or combined with the examples above. If your carbon monoxide signals very seldom try to be aware of atmospheric conditions at the time.
Check and consider these factors up to and including possible propane leaks (if you have) or a malfunctioning oil furnace or even a cracked furnace heat exchange will spill carbon monoxide into your home (something for the furnace tech to check into). If a carbon monoxide alarm goes off a second time and the cause is still undetermined then I would suggest having a qualified person (gas company / Fire Dept.) with a carbon monoxide sniffer test while the event is occurring.