Read the info and disclaimers that came with your UL listed co detector. They do not sound until levels are much greater than seems reasonable. Check out this link to a unit that sounds at low levels. http://www.coexperts.com/One.pdf
Yeah my Kiddie Night Hawk CO alarms have the digital readout. It reads 0 until the level is at or over 30 ppm.. Then a 70 ppm level has to be maintained for a specific amount of time (I think 2 hours) before the alarm will sound. The time shortens for the alarm as ppm increase. Anything under 30 ppm is considered no concern, by the alarm anyway If you have pilot lights running on your cook stove you could possibly have 10-15 ppm in your house all the time.