Most likely the security system. Many times while shopping for a new car
the things had dead batteries when I demo'ed them. They always blamed the
security system as well as some electronics that are never off (radio
controls memory, computer, etc.).
Well, you could disconnect one of the battery terminals and connect it
through an ammeter. Make sure the meter's rated for 10A at least and DO NOT
start the engine. Check the current reading when everything is off. It's
hard to say what a good value would be, but it should definitely be
significantly less than 500mA. (Maybe around 100mA or even less) If there's
too much current, you could start pulling fuses to find the culprit.
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