When you test drove were you test driving a dedicated test drive vehicle or were you trying new cars off the lot and dealer tries to keep each car under 15 miles?
I've purchased nine or ten new cars for myself and my wife over the last 30 years, and the only ones that I've had problems with were ones that had been used as demo vehicles at the dealers. Coincidence? Maybe, but I always insist now on getting a new car "right off the truck." Most dealers are willing to oblige if you make it clear that refusing to do so means losing the sale. Of course, that can mean waiting a while to get the model and color you want, but I have found it worthwhile.
My 2014 3 had 26 miles on it, but I put a fair number of those on it during my test drive. I don't remember what it had before my test drive. Probably about 20.