Radio has sound when key is out and door open

I had this happen in my Chevrolet Malibu once. I was at a sporting event, and had some down time. I sat in my passenger side with the door open since it was facing downhill. In my case, you can sit in the passenger side, and turn the ignition to get radio power/on. It will stay on, even after removing the key. It seems to only respond to opening and then shutting the driver side door (to shut it off). Passenger door open/closed does nothing.
That is my guess. I do not fix cars, I just pay for them. Try that next time? Good luck.