Very likely air spring leakage. Replaced both of my front ones, very doable if you are practicle and not excessively expensive, use arnott springs I got mine for about £250 each via ebay. Dont rush though, for both of my fronts they leaked occasionally to start with and then gradaully worsened over a couple of months. Try running car on level 3, mine only leaked on level 2 to start with. To diagnose cover spring in soapy water, fairy liquid, if it is leaking it will be obvious with the bubbles, obviously may not be that obvious if its an early/slow leak but I waited for it to be obvious on the cause before fixing.
don't drive car when its sunk to lowest, wait for the car to pump up before driving off. only slight risk of waiting too long is excessive work for the compressor as these can wait out too. Personally I did mine when they always sunk when left for a few hours on level 2 (able to see bubbles) but were not leaking on 3 so could drive about without worrying about excessive compressor work.
Do feel free to ask if you end up doing it yourself, there are a couple of crucial tools/socket bits you will need and want to know about before you get past the point of no return!! Also, if you go to a dealer I believe they have to use genuine audi parts but actually the Arnott springs are much better, loads cheaper and have lifetime guarantee so find yourself an independant garage/specialist.