...the drains in the front corners.
I drove down the road and as I went up the next hill I had a sudden influx of water - right into the controls for the sunroof/alarm receiver. This was how it must have failed the first time - nobody had thought to sort the leak out (I cleaned the electronics and now have a nice working remote lock).
The drains in the guttering under the sunroof must have blocked and allowed the usual seepage to pool in one corner of the car. When parked on a more level slope (level from left-right) the water must have drained through the free opening, but the one on the drivers side seems to have been blocked.
I'd tested it by pouring water into the guttering to check for leaks, but instead of flowing away, it pooled up - I had to quickly grab a cable tie and poke it down the drain hole to unblock it. Glug! It emptied - one more problem fixed! :)