Do you have the original style alternator with an external voltage regulator or has it been upgraded to an internally regulated 10si or 12si [systems integrated] regulator?

Older regulators where mechanical and newer ones are solid state. The number four connector on the regulator is ignition switch controlled off/on [brown], number three is voltage sensing [red] connected to the buss splice at the horn relay and always hot from battery, number 2 and F go to the alternator [blue and white]. If your voltage regulator is failing it MAY be allowing some current flow from the voltage sensing wire to the alternator. Or IF your ignition switch is not turning completely off it MAY be allowing some current flow to the regulator and allowing current flow to the alternator even when the car is not running and the system is supposed to be shut down. You could take the connector off the regulator and check the number four connector for voltage with the key off and on.

If you have an SI alternator the brown off/on wire will go to the number one pin on the alternator, or splice into the blue wire that goes to number one pin. The red voltage sensing wire will go to the number two pin on the alternator or splice into the white pigtail wire to the alternator. You can pull the brown[blue] wire off and check that voltage drops to zero when the key is shut off. If you have voltage to the brown[blue] wire when the key is off you have a faulty ignition switch or a short in the harness. If you get a voltage reading at the number one pin in the alternator then current could be flowing from the sensing wire through the alternator and out the number one pin to the idiot light in the dash, in which case a diode in the off/on wire would stop current flow.

Clear as mud?

Of course I could be wrong.

Be careful when using a sensitive multi-meter when checking for small volt or amp readings you can get a current from one probe into one hand through your body and out the other hand to the second probe. I had to release both probes on my meter to get an accurate reading. Really!