Remove the positive lead from the battery.
Place a multimeter set on ohms between the positive lead and the earth lead.
Then you can start checking things to see what is pulling the battery down.
Pull fuses one at a time, if the resistance changes dramatically then you will know which circuit the drain is on.
Disconnect the alternator, i have known them to drain a battery in a very short space of time,
In an ideal world the ohms reading should be as close to zero as possible, but on a modern car there will always be something
drawing from the battery.