Is your climate system Off? If it's not, and the temperature it's set to is warmer than the outside air, the car will fire up the ICE to heat the cabin air. I've found this even when the climate system is not in Auto and the A.C. is not on, so it's just in ventilate mode. So, to keep it in EV mode, after starting, immediately press the Off button on your climate system. Of course, then you'll be cold! Using the seat heater won't fire up the ICE.