[ChrisZX]

I noticed this a few times. It appears that if one puts the car into gear immediately after turning the knob to start the car, the car would start the motor while going through the check list on the dash. It's possibly a feature to provide power right away when electrical system is still being checked.

If I allow the check list to complete before putting the car in gear, the gas motor stays off until the appropriate drive mode is selected. Not sure if this is what the OP is seeing. Sounds close.

 

[ChrisZX]

It seems your engine started before you even touch the gear selector.
If it keeps happens before you select a gear, it may be time to get in touch a dealer. We've had our car for about 2 month now. Many short trips on electric only, and the gas motor stays off except the couple of times I was too quick to put it in gear.