BMW 523i Battery drain

I have a 1998 BMW 523i (I know this model is slightly outside your range of expertise but hope you may be able to help). The problem is that periodically, usually every few weeks the battery becomes flat and the car will not start. In this condition I just get a click and the engine will not turn over. It has had a new battery this summer and it has been checked by my mechanic and an auto electrician who can find nothing wrong. The drain on the battery when all electrics are off is well within the specified limit. I charge the battery and all is well again for a while when the problem reoccurs. This problem has occurred during warm summer conditions when lights and heaters are not needed as well dark cold winter conditions. This seems to be a very difficult problem to diagnose but just wondered if anyone has come across a similar problem.

Andrew, make sure the current draw with the car asleep is less than 40 milliamps. A common reading would be 22mA. The car should have this reading after the shifter LED goes out (about 16 minutes after last input). Make sure you are not leaving the key in the ignition when you park it. This will cause a dead battery. The ignition switch is a common problem that can cause the ignition to turn on by itself. Make sure the alternator is charging properly. Please make sure you have these items investigated.