In what way were the mirror covers incorrectly fitted? How would this only effect the drivers side, you stated the passenger side is quiet?
I also find it strange that the factory would do this and subsequent QC check wouldn't spot this. Just the one car in how many thousands built?
How would this only be noticed as vehicles passed and not during driving as the air flow surely would be very much the same?
Paying for diagnostics is quite normal if nothing is found, that said I have always had this waved by my dealership.