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.