I tried the car exhaust idea just now on my two First Alert zwave alarms. The first didnt alarm at all near the tail pipe and then I noticed I mistakenly bought a smoke alarm only
I grabbed the other I had bought which definitely was CO and smoke. It did alarm, but for smoke according to the LED on it. Tried again little bit later and no alarm.
Nothing happened in HG either time :-( Nothing on the widget, nothing in the event log for node 9 or 14.