Story on foxnews yesterday included a detail that the ATF inserted GPS tracking devices in these guns before the sale.
They knew/know where they are. How many other firearms have these GPS devices installed?
And where are they now?
Someone also tried floating trial balloon of RFID chips in those guns. Both have their attend technical problems
GPS units would require battery power that could run out before the gun reaches its destination. It would also have to transmit this data unless the idea was that it would be like tagging geese or something.
RFID would not require power but these chips have a limited interrogation range.
Moreover, both methods would require and RF view of the sky – any kind of RF shielding would block the signals making these tracking methods useless.