I know you said that you replaced the alternator but I'm not sure if the vehicle has a separate voltage regulator or whether it is inbuilt into the alternator so check the charging voltage across the battery with the engine running is in the range 13.8VDC -14.5VDC If it is less than 13.8VDC the battery will not charge and will discharge slowly as it will be helping the alternator to run the electrics in the vehicle.
Be SAFETY AWARE when you test as the engine is running!
How long does it take to go flat?
Have you checked that everything that can be turned off has been, e.g if the boot (trunk?) courtesy lamp is left in the Always On position it can drain the battery after a day or so. Also if there is any after market equipment that has not been wired via the ignition key power supply feeds but powered directly from the battery
Otherwise, with the engine switched OFF you may have to remove the -ve cable from the battery and place an Ammeter (function found in a DMM - digital multimeter) in series between the -ve post of the battery and the -ve battery cable and read the current flow. There should be a reading of about 100mA or thereabouts as there is always such things as an immobilser and alarm system (if fitted) and some other electronics that need to be kept powered.
If you read a lot more than this then you'll have to start pulling fuses one by one and watch the Ammeter until the current falls to a more acceptable level.
Once you find the fuse you'll know what circuits are being supplied and hopefully what may be happening.
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There are so many electronic modules on vehicles which can drain battery it is difficult to narrow down. I would need more information, like a timeline, do you park car and is it an overnight drain, or a few day drain as example. What you need to do is a process of elimination, start by not locking vehicle to see if it is your alarm system which is common to see drain battery if malfunctioning. Of course, check all your lights, door light switch failures can keep interior lights on draining battery. Did you check alternator output to insure your new battery is being charged each time you use vehicle. Have you checked batter voltage while disconnected from vehicle, and then check when connected, look for a voltage drop or if you don't have a meter, see if you have a small spark when connecting battery or a larger one, a good indicator of short circuit in vehicle wiring. Small spark is normal, larger one is indicator of short to ground. You can narrow it down further by pulling fuses to isolate,
par M Sullivan