if you decide to go to the garage and its not far and there is still some coolant in the bottle just top up with water as its around £17 for a litre from audi(good stuff)i got the same warning message on my a4 avant and found after removing the large undertray that the bottom pipe(nearside) leading to the radiator had a leak....(if this is your case)if you can get a flat headed screwdriver and lift the clip up thats on the end of the pipe(dont remove it) and try and push the pipe as far as it will go on, and at the same time close the clip(sometimes these clips pop up and cause the leak,mine was a fast leak)......if that does not work and it still leaks from there,then you have to buy an O ring from audi(common fault, thats why they will know which one you will need when you tell them where the leak is coming from...and insert it inside the bottom radiator pipe(make sure its flush.......now heres the hardest part getting the pipe back on while trying to close the clip with the O ring in place. i found it very hard.even then i dread hearing the warning light coming back on as uncertain if it will stay in place.regards