What I would first do is measure the actual drainage: switch off everything electrical in the car, then disconnect the negative lead on thebattery and connect an Ampere meter between lead and battery negative terminal. Make sure to select the highest Amp setting first and gradually switch to lower settings until you measure a current. Then pull fuses until you see the Amps go to zero. This provides the offending group and you can look further for the cause in that group.
Causes I’ve seen: alternator with a defective diode (like onakaw mentioned), trailer hitch connector with wet sand shorting the constant plus (interesting fault), light in the boot or under the hood/bonnet not switching off, Alarm system, bare wires shorting, water in lamp unit.
If the drain current is high it should not be too difficult to find
Best of luck and let us know when you find the cause. We’re always eager to learn 😉