When I started my car today, it asked for the radio unlock code. I entered the code and everything was normal - the radio worked for the remainder of the trip, and when I turned the car off and on again, the radio continued to work.
What would cause it to require the code, as I want to avoid issues in the future. The car was sitting in a (hot) parking lot all day, and there was no evidence that the battery disconnected. The car started fine.