Why is there no coolant in my ford focus?

I have an 02 Ford Focus. Checking the oil yesterday I noticed that the coolant had 'disappeared.' The oil was fine so no leaking into there and I can't see any leaks on the driveway.

As yesterday was a bank holiday I couldn't go to the motor factors to get coolant so I just topped up with a bit of water until I got a chance to go to the motor factors today.

Took the car for good drive to see if the car would over heat, but it didn't.

What could be wrong here?

If you were checking for fluid in the plastic "container" you can see, that is actually the expansion tank (coolant/water rises and falls in the tank as the system heats up/ cools down). Regarding coolant "disappearing" you can sometimes have a small leak in a hose (it will get bigger or blow the hose as it fails) or in the radiator which may be small enough that it just burns off and does not leave a puddle. That said, it you notice a sweet smell when running the car or for a while after you shut it off, you may want to get it checked out.

Same answer as above Good advice I say Old Chap.

There's a problem in the '02 Focus with the O-ring seal between the thermostat housing & the cylinder head. The housing uses an O-ring that is set in a groove machined into the housing. Problem is that on some of the housings, this groove goes too close to the back edge of the housing, & there's not enough material left between the groove & the back edge. If the narrow ridge between the groove & the back edge cracks & falls off, the pressure will slowly push the O-ring out & leak coolant. Then, when the engine cools & the pressure drops, the O-ring just slides back into place & everything looks okay. It's only like three screws to pull the housing off the cylinder head, so replacing the housing block is quite easy. You can only get a new housing from Ford.