As I understand it, it is illegal to leave your engine running when you
vacate the driving seat. The idea being concerns at the vehicle taking
off on its own, driverless.

These modern diesel electric buses use battery to get them to 2nd gear
speed, then start the diesel engine. When waiting at the bus stop the
engine is stopped, except often the engine will start up of its own
accord, whether the driver is in the cab or not.