If it is all drained out of the oil pan and radiator. You could put air pressure in one cylinder at a time with the valves closed. You might locate a leak that way. You could hear air in the cooling system. The real issue is the engine was sitting still at the time. Has to be a good size leak to gravity feed water to the pan. The oil line that was posted above is through a water jacket. And is often overlooked so it might pay to pull the side cover off and look. Water in the lifter gallery would be a dead giveaway.

Steve H