If you're filling the tank, 98 times out of a hundred you've got a leak. Mine hid itself by only leaking some of the time, confusing since I could drive it for days without losing a drop, then, out of nowhere, the tank would be half empty. The crack mine leaked out of was in the seam and as far as I can tell only sparated during rough city start/stop driving conditions. In addition, the water pump leaked which was caught in the underbelly tray. This of course hid itself very well. A bad water pump usually shows it's wear by the noise (rattles) it makes when it's just about done.
I started a thread on my "nightmare"
here