See
Help please!
Careful: It's not a real gauge. It's a fake gauge. It's more like an idiot light, really, cuz it has only 2 positions: Perfect and Too-late.
It's gotta be leaking to somewhere, even if it was 100% water. Maybe into the cylinders (warped heads)? Oops, that would be bad! Or your evap coil drips onto pass side floor carpet? Or maybe coolant is slowly dripping onto the engine block where it evaporates quickly?
BTW, you don't REALLY need special anti-freeze coolant. Using 100% water is perfectly fine, as long as you live in a place that never freezes (eg, Florida). If your winters get cold, then anti-freeze is needed only when parked in below-freezing temperatures.