Not Quite Getting The Idea

So, last week I roll up to the vietnamese sub shop to eat a big bowl full of Pho, and I notice I'm the first of the crowd to arrive. So I hang out in the Mazda listening to the radio. It's raining, so the car's dirty interior is totally preferable to the getting wet.

Anyway, I look over and notice the nice Prius next to me (ha, 50mpg? mazda gets 40). No big deal, lots of Prii(plural?) in Oregon (after all, it is one of the greenest states). I then notice the headlights are on, and the windshield wipers are going. But nobody was sitting inside the car!!!

It's not like a diesel engine, that used to need to be idled on fear of the fuel gelling up, or something else.