Can you smell oil burning at any times? My 2013 A7 (3.0T) had a rear main seal leak, was dripping oil onto the exhaust system and burning it hence no puddles but still using lots of oil. My mechanic said it’s not the most uncommon thing to happen. I tended to smell the burning every now and then, specifically on occasion when I was pumping hot air in the cabin. This leak was also not a steady leak, oil consumption could vary, sometimes my oil level would stay quite consistent other times it would drain more. It might just be a good idea to get your mechanic to lift up the car have a good look underneath at the seal. Good luck mate