Sometimes a faulty door switch for the interior lights could cause the interior light to stay on and drain your cars battery even when the car is off.
Or your headlights may fail to shut off like they're supposed to or you have a cell phone charger left plugged in and the 12 volt outlet didn't shut off.
Even a trunk light switch could cause the trunk light to stay on and allow the battery to drain overnight or when you're parked somewhere.
If none of that is the problem and your battery is good then the other most common culprit of a battery draining overnight or when the car is shut off is a bad alternator that has a bad diode.
When your alternator has a bad diode it causes the circuit to continue charging even when the vehicle is shut off and the alternator is no longer spinning.
When the circuit continues to charge due to the bad alternator diode the vehicles battery will drain as a result of that bad diode.
When that happens you need to replace the alternator with a new one.
An alternator can still test just fine as charging but some tests may not show up the bad diode on the alternator.