What you're noticing
The furnace kicked on sometime around midnight, same as it has every night since the weather turned. Nothing sounds different. Nothing smells different. Maybe someone in the house has had a dull headache for a couple of days, or the dog has been sleeping more than usual, and you've chalked it up to the dry winter air that comes through the Ozarks this time of year. That's the problem. There's nothing to notice. Carbon monoxide doesn't announce itself, and a furnace that's producing it can run for weeks without a single outward sign.
Why Nixa homes are built for this to happen quietly
A lot of housing stock around Nixa and the greater Springfield area dates from the building boom of the 80s and 90s, with furnaces that have been through fifteen or twenty winters of Ozark freeze-thaw cycles. Older heat exchangers crack from that constant expanding and contracting, and a cracked exchanger can leak exhaust gas into the air your furnace is blowing through the house rather than venting it outside. You won't see the crack without taking the furnace apart. You definitely won't smell what's coming through it.
Newer homes aren't exempt either. Tighter construction, the kind built to modern energy codes, holds heat in well, but it also holds in whatever leaks out of a furnace cabinet. Less outside air exchange means a small problem has fewer ways to dilute itself before someone breathes it.
More on this from Redeemed HVAC.
What the season does to the equation
Southwest Missouri winters aren't brutal by national standards, but they're long enough and cold enough that furnaces here run hard from November through March, with plenty of nights dipping into the 20s and below. That's months of near-continuous cycling on equipment that then sits idle and neglected all summer. Vents get blocked by leaves, nests, or snow drifts against the house. Flue pipes rust through at the elbows. None of this shows up as a smell or a sound in the living room. It shows up as a number on a detector.
What you can actually check yourself
Walk around the outside of the house and look at where the furnace vent exits, usually a pipe through the wall or roof near where the unit sits inside. Make sure nothing is blocking it: no leaves, no ice, no wasp nest. Check your furnace filter and change it if it's been more than a season, since restricted airflow makes a furnace run hotter and stress components it shouldn't be stressing. Look at the flame if your furnace has a visible burner window. It should be steady and blue. Yellow or flickering flame is worth paying attention to.
Beyond that, you're guessing. You cannot see a cracked heat exchanger, you cannot smell carbon monoxide, and you cannot reliably tell from the thermostat whether the furnace is venting properly. This is where owning a detector stops being optional advice and becomes the only actual line of defense you have.
Where it stops being a homeowner job
A carbon monoxide detector that alarms means get everyone outside and call for help immediately, not open a window and see how you feel. Low-level, chronic exposure is the more common and more dangerous scenario around here: a detector reading that creeps up over weeks, not a sudden spike. If your detector has ever gone off, even briefly, even if it seemed to resolve on its own, that furnace needs a professional inspection before it runs again. Furnace repair in Nixa during the cold months is routine work for the people who do it, and a combustion analysis with the right equipment will tell you in minutes what no amount of looking and sniffing ever could. Don't wait for a second alarm to find out the first one meant something.
Put a detector on every level of the house, test it the same day you change your clocks, and replace the unit itself every five to seven years since the sensors wear out quietly too. It's the cheapest, least dramatic thing you'll do for the house all winter, and it's the only thing that actually works.