I only had a moment of panic when the shuttle driver at the airport told me that I had left my car on as I was about to step off the van to enter the terminal. I did not have much choice. I could give her my keys or miss my plane if I went back.
Having been a loyal customer of this lot, I was not concerned. I trust the people working there. They treat their customers right. They give newspapers upon arrival and water bottles when you depart. They load the luggage from your trunk directly onto the shuttle.
By treating me right over the years, I was confident that the running motor would be turned off and my car would be there (with no additional miles) when I returned. It was.
Building relationships takes trust, treating others right and doing what you say. I do not recommend leaving your car on to test this theory. However, I do encourage you to create loyalty with both your customers and vendors.