Same issue here. 18 Atlas SEL. Last November, coolant light came on, tank was a little low, scheduled service, they said that there was a bad seal at the Water Pump, we'd have it back the next day. Called me later that day and said it was a larger issue, and they needed more time. What I was told, the engine when assembled at factory, the head bolts were not torqued to spec, that there was a small batch of these that had this happen. They rebuilt the top half of the engine, new head gasket, new Water Pump, all of that, had the car for over 2 weeks.
Got car back, few months go by, check engine light comes on, take it in, there are issues with the Catalytic converters, so they replace those, and most of the exhaust.
Fast forward to last week, I get in, Red coolant light comes on, check the expansion tank, its totally empty. Put some coolant in, and scheduled service, they pressure tested it overnight, and found no leak. I've never smelled, or seen any coolant on the ground. We park on the street so its not always easy to tell if something is leaking but the service tech told me that "These VW's run hot, and there are release valves that when they get too hot release coolant". I called BS, its a closed system, if its losing coolant, its leaking somewhere. Since I don't see or smell any, my assumption is it's leaking internal to the engine and getting burned off in the exhaust or something. My 18yr old BMW that I do all the work on never needs coolant, doesn't leak either.
This is my wife's car, and I'm seriously considering getting rid of it. Warranty is great but with this many major problems in less than 2 years its frustrating and I don't want her and our kids to get stranded somewhere.
My thought is that the leaking only occurs when the engine is at operating temp or under load. Does anyone know if there is a way to pressure test in that scenario to prove this out?