I recently purchased a 2005 A4 3.0 Cabriolet with 64k on it. Car is in great shape but last weekend I drove for about 2.5hrs and the temps were in the 90's so I had the A/C on and was doing at least 80mph all the way. Next time I start the car the coolant warning goes off. When I check the coolant level in the tank its well below the min line. I mean there was about an inch or 2 of liquid in the reservoir. I have not noticed any water leaking in the driveway. I checked the oil and it looks fine. the coolant guage goes to far left when I turn off the car and centers when I start the car and drive it. any ideas where I could be loosing coolant?

 

Check carpet area near the base of the evap box, in case the heater core is leaking. The carpet pad can absorb quite a lot of coolant before you notice it.

My '05 3.0 had a coolant leak when I got it, but the drip was obvious, and from the rear of the engine. In that case, the plastic coolant distribution piping under the intake manifold was where the problem was. Getting it out and the new one in was fairly involved; with the intake manifold off the thing is plainly visible, but disconnecting and moving enough stuff to actually change it was quite another.