CO-meter indicates 10ppm of Carbon Monoxide when I turn on Cabin heat - should I be concerned? Our Cessna 172P is equipped with a digital CO-meter rather than the traditional Quantum eye carbon monoxide detector.
Today I was flying along in winter conditions, and naturally used cabin heat. During the flight, I heard some occasional short beeps, and couldn't figure out what it was. I looked at the CO-meter which has a red alarm light, but that light was definitely off. 
On the ground, after shutting down the engine, I heard another short beep, and it definitely came from the CO-meter! The digital readout was showing 10ppm.
During post-flight, with the door open, the readout dropped to 0 ppm.
I routinely check the CO-meter battery during preflight, so I'm pretty sure the beep is not a reminder for a dead battery.

According to these sources: http://www.coheadquarters.com/ZerotoMillion1.htm , http://library.thinkquest.org/10121/textonly/exposure.htm and wikipedia 10ppm is nothing to be concerned about with exposure over a two-hour flight. 

Does anyone else have a digital CO-meter and can tell if a reading of 10ppm is normal with cabin heat?
Or should I be concerned our exhaust or heat exchanger is faulty?
The short answer is you should be concerned.  

Although 10PPM of carbon monoxide doesn't pose an immediate health risk for short term exposure the fact that the level is rising when you turn on the cabin heat &amp; falling when you ventilate the cabin (by opening the door) indicates you may have an exhaust leak, and those tend to get worse over time.
You should have your shop open the heat muff &amp; inspect the exhaust system for cracks/leaks before your next flight (or if that isn't possible avoid (or limit) use of cabin heat &amp; defrost &amp; monitor the CO meter as part of your scan until the exhaust system can be checked).

You should also check your CO meter: electronic CO meters have a finite life, just like chemical spot detectors (though they generally last several years) - your detector's manual will specify a replacement interval. As the sensors age they may give inaccurate readings.



A note about ground operations - Electronic CO detectors are sensitive enough to occasionally generate transient alarms during ground operations due to exhaust gasses entering the cabin. This can happen during summer or winter.
As long as the levels are low and the alarm clears when you're moving (taxi or during climb-out) this is probably not a problem, but you should know what is "normal" for your aircraft.