The only thing I could think was an 'overspeed' warning, I know the C2 has this and it beeps to tell you if you go over a set speed. I can't see that it would be something that nobody has ever noticed and I also can't see that it would have that alongside a 'speed limiter.'
Edit; Just read Arrans post! Great minds lol!
As I suggested in the pub, it is probably something connected to the alarm from what you described it seems like the noise when the alarm has gone off while the car was locked. I can't understand it at all!