We installed both the old fashioned black dot card CO detector and (this past week) a digital detector with visual and audio alarm on our recently acquired RV6A. Interestingly, this past Sunday the digital detector rose from 0 to 15ppm after about a 10 minute take-off/climb. Both the heat vent and fresh air vent (nearest the detector) were partially open (I was a bit warm - son was a bit cool) during this time. After seeing the 15ppm reading I then closed both vents and the detector dropped back down to zero within a few minutes. I then experimented by turning the heat fully back on in cruise flight for about 15 minutes and the detector remained at 0. I then turned off the heat and opened the fresh air vent - same story - detector remained at 0. Stayed at 0 for the remainder of 90 minute flight with heat on and outside air occasionally partially open. This has us scratching our head and wondering if the dynamics of our taxi/take-off/climb somehow caused some CO to get drawn in the fresh air intakes - but subsided in cruise flight. Anyone have any thoughts/insights?