You should be OK you have only gone a little over 2 weeks over on the Costco Best Before date, I don't know how Costco determines these BB dates. It should be determined by the (kill date )processing date marked on the meat establishment's beef box. If I buy a whole striploin etc. from Costco I will take it home and wet age the product for another 1-2 weeks depending on the appearance of the product so that's roughly a total of 4-6 weeks total wet aging. We have cut wet-aged beef when I was meat manager in the stores which was approx 50 -60 days but it loses its bloom and shell life very quickly unless it is packaged and frozen or cooked right away. Storing the meat in your home refrigerator should be not less than 38F for optimal aging on cryovac beef. As everyone has mentioned above the only real test is the smell test, but when you do open the brisket up and find it is ok I would advise you to cook it that day, as it starts to discolor very quickly and may take on an off taste the longer it is out of the package.