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AC Problem on 1999 MB ML320

I have a 1999 ML320 with 203,000 miles on it, runs great, no mechanical problems. I use a local indi mechanic who really knows Mercedes. I had a problem with the AC, went out recently on a Sat night great cold air, Sun morning nothing. Mechanic checked system found no refrigerant in system, recharged, added dye let run for 90 min. checked for leak found none, let car sit for 4-5 hours thentook it for about an hours drive both city & highyway speeds, AC on checked for leaks , again found none. Have been driving it for 3-4 days AC good & cold, still no leak showing. Where did refrigerent go?? He has no idea. Do any of you??