So the header tank was empty, interesting that it wasn't before you washed it and it is now ?
So, there are a number of places to check for leakage before we go to the full blown head off job, and not all of them give you a nice tell tale puddle on the ground and a leaking Td5 koppakking almost never gives you white smoke out of the exhaust.
So,
- heater matrix, you will need to look hard, but under the heater housing next to the right hand wing you will see some pink deposits, these will dry or dribble down between the wheel arch liner and wing and get lost.
- the fuel cooler, on the right hand side of the engine under the black acoustic cover below the inlet manifold, it leaks at the bottom the crimped joints front and rear and drips onto the engine mount.
- the water pump, behind the power steering pump, looking from underneath to the left look for pink residue around the casing.
- radiator, under the fan cowling which you remove, on the top left is the retour slang which rubs, and the top left of the radiator also leaks.
Finally the only sure and true way to test for kop leakage is a CO test, if it turns green, she's preganant
(left is the left of the car, right is the right of the car, never the way you are looking at it)
Rob