Last winter I experienced flat batteries with identical symptoms to those experienced by ST, Eltham. I have a '97 Golf TDI, which I have had for nearly six years. During the very cold spell last year I had a flat battery after the car had been standing a few days. As the battery was on the car when I bought it, I simply got on a bus and fetched a new one. Two days later it was flat. I charged it, and this time put a multimeter in series before connecting up. It was reading zero amps so no leak. Finally I tracked it down to the door locks.

Some moisture had got in the doors behind the trim and this was freezing. The resulting ice was preventing the locks closing completely, although everything looked OK and the doors were locked. This prevented the central locking motor shutting off so current was still flowing through a motor, flattening the battery quickly, but of course leaving no trace when the car was unlocked. The only way to test this I can think of would be to use a meter when the car was very cold and locked, but this would still be a question of luck depending on temperature and the exact position of any ice.