Hi,
HikaSerial link the serial with the user. In the HikaSerial backend, you can see in the serial listing (and in the serial form) the user name (with a link to his HikaShop account).
Careful: There is an option in HikaSerial 1.7.3 in order to do not make this make.
After that, in HikaShop the coupon code, when use, is stored in the order.
We can see to improve the interface in order to improve the access to this information, like a listing in the product code form.
It could be possible to search for the product code in the order listing (and add the column to display it), so that can be a solution.
It could be possible to modify HikaSerial in order to display some information in the serial form about the serial.
But it will require some development because we have to trigger the right plugin which have generated this serial (I guess using the package configuration, it will be easier than put sort of "tracker").
Regards,