Yes, thanks, I picked that up from searching yesterday, but this noise was continuous for 15 minutes... we had started doing other things after we came back when my wife said 'the car's still making quite a noise', and it continued when I came out to see.
As noted elsewhere, if you dont lock straight away it goes on for longer, and it you double-lock it stops after 30s-ish, as I described, and I definitely double-locked when we returned this morning.
Having measured the current drain yesterday while it was whistling, I am really concerned that it will flatten even a new battery in a day.