Your vehicle is equipped with a Retained Accessory Power (RAP) feature which will allow the radio to continue to work up to 10 minutes after the ignition is turned to OFF.
Your radio will work when the ignition key is in ON or ACC. Once the key is turned from ON to OFF, the radio will continue to work for up to 10 minutes or until the driver's door is opened.
Your vehicle's sunroof and heated seats, if equipped, and the power windows also work this way. They will turn off after 10 minutes or if any door is opened.