Is the PIC still responsible for everything when they leave the plane and a co-pilot is staying on board? Are there differences between operational reasons for leaving the plane (doing the walk-around) vs private ones (going to grab a snack in the airport during a layover)? Does the PIC have to specify someone to be responsible for the plane while they’re gone?
If there is no international rule but airlines have rules in their SOP, what are those rules generally?