Where did my coolant go ?

[Stefvan]

I got a red flashing light on my JX shortly after setting off this morning, pulled over and checked the top up tank and the oil level. Carried on flashing but didnt majorly overheat, when I got home I found the header tank was half empty. I have refilled it with approx 1 litre coolant but wondered where its gone... and why it didnt draw what it needed from the top up tank. No trace of oil or contaminants although the coolant is generally murky blue. I tried to get a replacement vw dalek header tank cap, (although the valve in it seemed to work ok ie i could blow but not suck air through it.. ) do you think a new one will cure the problem ? I have never had to top up the top up tank since i got the van last year.. no obvious leaks anywhere.

tia
Stefvan

[Aidan]

leak is sufficient to cause loss of vacuum on cooling hence system isn't topping itself up or the darlek cap is fobarred or the engine has a head gasket leak water jacket to cylinder
big leaks that aren't noticed are usually on the front to back pipes or over the exhaust