Just a thought gness. Going back many years cars had dynamos instead of alternators, with a warning light to show if they were not functioning properly. When alternators first came along I seem to feel there was a problem with the working of the warning light - if the warning-light bulb failed the battery could discharge through the alternator. This is all going back a long time but, if the problem still exists, it could be that which caused your battery to go flat over the other weekend. The message really is to make sure that your alternator warning-light comes on when you turn the ignition on and goes out when the engine starts. Your garage may know if it's still a problem or if a proper fix was found for i.