Electrically speaking, a battery will discharge faster over a load with least resistance. Something is draining the battery by offering a relative connect to ground, even if you shut your engine and remove the key. That said, figure out a way to measure current at various places while everything is off. Watch for wires touching each other or the body/chassis.
Now, if the draining is happening while the car is on, then your charging system isn't keeping up. Sometimes some relays can also cause a diversion of power to protect other components. Sometimes a module such as the ABS could eat up more energy thus offsetting compensation from the charging system.