Hello,
I know this reply is so late given when this thread was first started, but for whatever it might be worth, I also purchased a Balenciaga perforated mini city bag from an official, authorized Balenciaga Outlet store. (I'm in Southern California, USA, and purchased my bag about 2 years ago at the Cabazon Desert Hills Premium Outlets). I inquired about my bag missing the mirror at the time of the purchase, and the sales associate, who was very nice and super helpful otherwise, informed me that the bag did not come with the mirror. He also said that some bags that are produced for the outlet stores have minor differences from the ones that are sold in the traditional stores. A couple other people have mentioned throughout the forums that they experienced this at Outlet stores as well, so I also wanted to chime in. I've been looking to purchase a stand-alone mirror to go with this bag ever since. I'm so sorry that yours didn't come with a mirror either. But, in a way, it's comforting to know that someone else has experienced this issue. If you see this, I hope it helps, if only for peace of mind.