Current draw is what you need to be looking for. Get a cheap multimeter remove the negative terminal from the battery and place the meter in between on Amp setting, or get a cheap clamp meter and just pop it over the negative terminal.
From there you need to see what the drain is, and to locate the drain remove and refit items/fuses one by one until you see a noticeable change.
You could find this will be anything from a boot lamp not turning off, an old tracker you never knew was fitted, a faulty alternator or any of your in car entertainment.
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