Checking for small electrical drains (as mentioned) is a good idea. Seems like there was one time that the trunk light switch failed on our '66 Chrysler that caused a low battery condition until I finally found it. Being that it's a spring loaded contact-to-ground switch, it came apart and completed the circuit and turned on the trunk light. The simple fix was to just unplug it until a new switch was ordered in by the dealer. Others that have a mercury switch might have a similar problem.
Other thing is that a diode in the alternator might be not up to snuff. All it takes is one. Might not be failed enough to cause a low charge rate, but when they fail it can cause a low battery issue pretty quick. All it takes is a continuity checker to see if current flows both directions instead of just one.
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