Further research
The random music only ever seems to happen when the 660 is in the bike mount, and not in the car, which made me think it might be a foible of the bluetooth.
This weekend, I discovered that the bluetooth receiver that I attach to my headphones has a "play" mode; if the on-off button is quickly pushed when the unit is on, it will cause the 660 to activate the media player and play music. If it's pushed again, it stops.
I reckon because the sender lives in my pocket, as I move around on the bike, the button occasionally gets pushed, causing the 660 to play, and then gets pushed again, causing it to switch off. At least I think I know what's causing it now, which isn't a huge amount of help as I can't turn the play mode off...
Ho hum.