I woke up around midnight and couldn't be absolutely sure I'd turned off the tap that feeds the horses's water butts so- after trying to locate the memory and failing- I got up and traipsed down the drive in my pyjamas and slippers. It had stopped raining by then but was still cloudy- so no stars.
The tap was off.
I can't think of a way to stop this happening. I could keep a log book, I suppose- but one can forget to write in a book as easily as one forgets to turn a tap.