Loosing water from the Rad

[Maz]

On average I need to top up the water on my VW Passat 19tdi once a week.

I know it is low because the heater starts throwing out cold air when the temp dial is bang on the centre

Today I got home and left the engine running with the fan on full blowing cold.

Opened the rad and it was near empty got the hose pipe out and gently filled it up.

Put the cap back on, left the car running while i wrapped the hose pipe up.
Checked the heater and it was still cold.

checked for any leaks and found nothing, got called into the house for a few minutes but when I came back out, it was like there were 2 taps either side of the plate number which had been left on

I turned the engine off and tried to see where it was coming from. No luck, what could it be.

I love the car but each time something goes wrong it seems to cost over £200!! Please tell me this will be an easy fix