I can't help you regarding the US. I live in Germany and there's a law (§15 StVO) that says "The driver of a vehicle must take the necessary precautions to prevent accidents and disturbances to traffic when they exit the vehicle. Vehicles also have to be secured against unauthorized use."
I had a quick google and I found mentions that you will get fines for this. I did not find an example of something bad happening with an unlocked car.
I had a quick google and I found mentions that you will get fines for this. I did not find an example of something bad happening with an unlocked car.