“How would they be able to track the 9mm Glock purchase down to a gun shop in Roanoke, and interview the owner by early evening (6-8 hours afterward), if he’d filed the numbers off?”
Many firearms have “secret” SN’s due to anti-gunners demanding design changes. The design demands usually include a magazine disconnect, loaded chamber indicator, secondary SN, locking device.
Or, the earlier report of obliterated SN was wrong, which is the most likely.
The guy had receipts for the gun purchases in the bottom of his backpack. They didn’t need the serial numbers.