How do you know it leaked out of the drain plug?
Did you see drips coming from the plug area?
A leak from the water pump or water pump housing would end up in the middle front of the car.

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Current Mercedes
1979 maple yellow 240D 4-speed

Gone and fondly remembered:
1980 orient red 240D 4-speed

Gone and NOT fondly remembered:
1982 Chna Blue 300TD

Other car in the stable:
2013 VW Jetta Sportwagen TDI / 6-speed MT