Also had a mysterious jingle today, on the M6, driving to Hampshire from Cumbria, a type of audible warning we hadn't heard before, we were also puzzled.
But reading the above, I think I've cracked it, it may have been a rear seat belt warning, with a suitcase on the back seat and possibly uneven road, putting occasional additional g-force into the seat sensor.
Perhaps I'll try driving the car from the back seat, lifting my glutimus maximus as I drive, simples? or can anyone suggest a more suitable perhaps simpler test ??