From the owners manual:
"The car will select the appropriate source of power depending on the drive mode you select.
As a result, the engine will automatically start or stop as needed to either charge the battery or provide supplemental power.
Under certain circumstances, the engine may turn on or, if it is already on, it may not turn off.
● You are going uphill or accelerating aggressively.
● The climate control system is in heavy use.
● The Ambient temperature is too hot or too cold.
● The High Voltage Battery state of charge is very low.
● The vehicle is running a system check.
When the engine initially starts (between the time the POWER button is turned on and turned off), the vehicle conducts a system check.
● While the check is being conducted, the engine may periodically turn on and off. This, however, is normal.
● The curved blue line in the POWER/CHARGE Gauge will not appear during the system check (EV indicator may still turn on).
● Once the engine starts, it will continue to run until the system reaches operating temperature.
The curved blue line in the POWER/CHARGE Gauge will reappear once the system check is completed."
Point is the engine may turn on at unpredictable times. This is a plug in hybrid, not an EV.