My 2001 Focus TDDi (130k miles) is rapidly losing water but I don't know how or where from.

Here's the story. Owned the car for about a year with no issues. Never needed to top up water and never over heated.

About a month ago she stalled while driving at speed so I looked at the gauges and noticed i had overheated. Pulled over and allowed to cool down and topped coolant up with 2 ltrs of water I happened to have in the car.

Once home I topped right back up with water/antifreeze mix.

Since then it has needed regular topping up. By that I mean the temp guage starts to rise above normal after around 50 miles or so and I need to refill with around 3-5 litres of water.

The thing is, there is no sign of water leaks anywhere. No damp patches where I have parked, no dripping from underneath when hot or cold. The only evidence is where water came out of the cap on the original overheat.

I did think maybe the head gasket had gone but the car runs and idles fine and does not seem down on power which I would suspect if the gasket was at fault.

Any thoughts? Is it possible the head gasket has gone without showing any other symptoms?

Thanks for any help you can give.