After years of lurking, I've finally got a reason to post an issue I may have.
My wife was driving out 2007 S40 last week and was listening to the radio. She decided to make a phone call and turned the radio off (by pressing the volume ****/power button), and approximately a minute later, it turned itself back on, but the word "ALARM" was displayed on the stereo display and a weird, alarm-like noise was coming from the speakers (she can't remember what it sounded like). She immediately pressed the volume ****/power button to turn it off, and it ceased the weird behavior. It hasn't happened since, she said, but I was a little worried about what this might be. My gut is telling me it's some kind of electrical issue, but I've never seen this happen before, so I'm not quite sure.

Thanks in advance!