Does sound like bad luck. Do you have an equivalent BMW loaner throughout this (you should have)? Lemons are not that common. It's unlikely but entirely possible that one person ends up with two lemons in a row.

One more reason we should always carry a digital camera or keep a cheap one in the car (the simple ones are very cheap these days if you don't have one on your phone).

If you get it back and it smokes again get as many photos as possible or use video if the camera supports it. If the smoke wasn't too bad, I'd also chance opening the hood (be careful when opening) and try to take photos both above about below the engine compartment. Should narrow it down and provide enough evidence of a serious problem. Chances are they will figure this out though - with that much smoke, there has to be visible damage somewhere.

Sorry man.

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1995 BMW M3 (Track prepared)
2006 BMW 530xi (Wife)
2011 BMW X5 35d (Tow/Daily)