See my response to "Losing antifreeze". It might be your answer.

 

These cars are known for the failure of the heater hoses that run under the intake manifold. The heat in there can dry the coolant before it runs onto the ground or it will run off the rear of the engine where you are not looking for it. The other place is the thermostat housing which is plastic. Ther eis an update4d housing made of aluminum.Next, the heater matrix could be leaking and the coolant will run out the AC overflows near the transmission, but you would smell this inside the car strongly. Thereis an outside chance that the headgasket could be leaking, so look at the dipstick and make sure it looks likeonly oil on it. Finally, the heat exchanger in the radiator for the tranny can leak and let coolant into the tranny lubricant although this is very rare.

 

Sometimes, it is difficult to find a leak. It maybe leaking due to running warm and
overfilling the reservoir. If the car is running warm, the temperature gauge won't show it until it is too late. Check that both radiator fans come on.
If the fan(s) don't run, check the 30 amp fuses in the engine compartment on the
left side. If the fuses are ok, disconnect the battery and be sure the fans spin
freely. (They sometimes rub against the shroud).

If the car is indeed cooling properly, try running it with the plastic engine covers off
and look for steam. A small leak will vaporize. The Jaguar plastic lines are not the best, but they are under $30 to replace.

If the reservoir empties in a few minutes, you may have the dreaded head gasket leak. If the cap is off and the car is running, you will see exhaust bubbles or a
major amount of exhaust just push the water out.