If you have nothing in the circuit like a radio for instance that might have a capacitor in it, the only place I can think of for a significant leak would be in the charging circuit. In an alternator it is usually leaking diodes in the Alternator. In a generator it is usually leakage in the regulator. Leave the battery hooked up and disconnect the power to the Alternator/regulator circuit. Put your multi meter in there on milliamps and see if you have current draw with the charging circuit disconnected.