Your vehicle has a high voltage to low voltage energy transfer feature that keeps the 12-volt battery charged by the high voltage battery. If the 12-volt battery level is low, the high voltage battery transfers energy to the 12-volt battery when the vehicle is off.
12-volt battery depletion can still occur if the high voltage battery state of charge is low. See
Preserving Your High Voltage Battery
Always take care not to deep discharge the 12-volt battery as it may cause premature wear.