When the cooling system has not been serviced regularly or when someone fills the system with water after a leak has been repaired, corrosion inside the system can occur. Core plugs are made of steel (unless they have been replaced with brass or stainless steel). They commonly rust out and begin to leak.
The core plug behind the flywheel was leaking.
Core plugs are usually located on the sides of the block. Sometimes they are found on the front and rear of the block, too. Use a mirror and a flashlight to inspect core plugs for signs of rust or leakage.