After few months of using a digital CO detector, so far I can say that most of the time you smell "engine" inside the cockpit, chances are you are getting few ppm of CO, usually at engine startup (some exhaust designs seals off after expansion due to gained temperature) it can be strong in some aircraft.
Another common high reading is when doing slow speed exercises, or transition from Hasell checks to stalls or practicing PFL. And of course when applying hot air to the cabin if the system is not correctly checked by the mechanics.
Now my habit is whenever we start the engine we keep a window, storm window open to freshen up the air for few minutes, the same when doing the slow exercises if you the airplane poh allows us to open the window in the air.