Engine Running Full Charge

You probably heard the AC. It's quite loud on max.
As mentioned, opening the hood guarantees engine on if the vehicle is on. It's for safety and certain diagnostics where you need the engine to run (but it's normally off at idle on a volt)

wizard_of-ahs said:

From my 2014 manual:
Electric Mode
In Electric Mode, the vehicle does
not use fuel or produce tailpipe
emissions. During this primary
mode, the vehicle is powered by
electrical energy stored in the high
voltage battery. The vehicle can
operate in this mode until the
battery has reached a low charge.
There are some conditions when the
battery charge is high enough to
provide Electric Mode operation, but
the engine still runs. They are:
. Cold ambient temperatures.

. "Hot or cold high voltage battery
temperatures."

. The hood being open or not
completely closed and latched.
. Certain high voltage battery fault
conditions.
. Engine Maintenance Mode or
Fuel Maintenance Mode
being run.

So apparently the high temp will trigger it

Click to expand...

Eventually, but not in OPs case.
Full battery means he just left a plug and the battery is normal temp.
It would take hours in the ambient heat to get to extreme highs, especially with the AC running.
What's described there is more of a critical mode when the AC is unable to cool the battery more than it's heating up, or AC is disabled entirely. It will shut down (most of) the flow of electricity to/from battery to protect it.
The same happens in extreme cold (which takes about 10 hours at -25C to get to, from a normal temp battery if no heating applied) where the computer shuts down most battery flow to protect it and uses the engine for main power.

Such situations do not happen in every day use, and certainly not within an hour of being "exposed" to the extreme temps.