I had this problem i.e. battery goes flat after 3 or 4 days of not using my car, changed the battery, checked for lights staying on, disconnected the radio, alarm, etc
Then unplugged the alternator, left the car for a week plugged the alternator back in and the car started first turn of the key It turns out the alternator had in internal fault where it charged ok but, a fault with the regulator inside the alternator allowed the power to drain away.
New alternator fitted fault gone
Maybe worth a try