Had a strange experience today. I pulled up to the shops and got out of the car and locked it. Whilst in the shop the alarm went of so I unlocked the doors to stop it and continued to get what I needed from the shop. I came out and got in the car and the radio had come on, strange as I thought you could not have the radio on without the key in the ignition.

 

You can have the radio on without the key in. When you switch the engine off and remove the key the radio will also switch off. But, if you press the ON switch on the radio again it will come back on. I found this out when the chap was fitting my mobile phone car kit.