My colleague at work has a 98 FL 1.8 and kept saying the water level slowly went down and he had to keep topping it up! When you got in the car, you could smell the anti-freeze but checking under the bonnet, you couldn't see it leaking out anywhere!

After a frustrating month or two, he finally tracking it down to a leaking radiator! I helped him change it and at the bottom of the old radiator, it had corroded through in a couple of places close to where the fans were mounted and was constantly weeping out. The heat of the radiator was enough to evaporate the liquid off so that the dampness couldn't be seen but it was only after removing the fans, the telltale antifreeze crystals were found.

If anyone has a similar problem of slow water loss, it may be worth having a look at the radiator itself.