Was there any reason given on the dash or in the MFT screen where it shows the icon of the car and the energy flow, or did you look there?
Reasons for the engine coming on are:
a) using defrost or heat
b) HVB being full and slowing down with other than brakes (car is smart enough to stop regen while using brakes and not start the engine).
c) Alot of little engine uses causing oil maintenance mode
d) engine being warm from earlier and just churning it for a minute to keep it from totally cooling down.
e) hard acceleration to get out of trouble or merge quick
I have experienced a, b and d, but never for c or e.