Have you considered that you may have electrical circuit active when you shut the engine down, which would be drawing down battery reserve. If the battery is continuously drawn down while car is parked, this then of course causes the alternator to go into full charge mode for possibly extended periods of time until battery is replenished again. This would reduce life expectancy.

A better method would be to use a meter so that you can measure any draw.

Simple test for this would be to disconnect ground cable and with a test light, connect one end to the negative battery post and the other to ground. Make certain switch is off and doors are closed.

If light illuminates, could be anything from a glove box or trunk light to a bad voltage regulator to an excessive draw from an alarm system to a radio wired to accessory side of ignition switch,,,,etc.

If a draw is detected and you do not know from where, an initial test would be to remove fuses until light goes out,,,this would be the guilty circuit. Would not catch a defective voltage regulator or devices on circuit breakers.