Having seen the "voluntary" federal registration system first hand, I can tell you that any firearm transaction that goes through NICS is recorded in some sort of way. I have seen the result of such a system in the form of a spreadsheet that showed all of the firearms with serial numbers that an individual purchased over the course of two years at multiple locations. How they (the ATF) assembled that information is neither here nor there. The fact was they were able to get it quickly and with few errors.
Out of curiosity, what were the circumstances surrounding this?
The "how" they assembled the information is actually important. It sounds like you may have first hand proof that federal laws are being broken by federal agents. If you do, I urge you to contact your congressperson and the media and let them know.
They audited one FFL, but somehow were able to discover what other places that individual had purchased firearms from. If that is not the result of recording NICS checks and geographic location, then I don't know how else they could have done it.
For reference, most dealers file the 4473 alphabetically by last name. If I were an ATF agent, and I needed to know what guns Person X owned, I could probably look at their address, draw a 40 mile circle around it, and pick up my phone to call all of the dealers in that circle.
If it was really important, I'd probably have another ATF agent help me with the phone calls.
As a dealer, if the ATF called and asked me which guns Person X bought, it would be about a 5-8 minute phone call depending on how many guns I had to read off.
How many gun dealers are there in your area? With a couple of people and a couple of hours it doesn't seem that hard to me to put together a mostly accurate list, with serial numbers, of a specific purchaser.
What if that person had moved around a bit? Although it might take more time to get warrants and cut some red tape, getting your past addresses would be as simple as checking your credit history. Then you repeat the above task in those areas.
In other words, what you describe could be accomplished by good old fashioned police work.
If the person happened to travel a lot, and had purchased a shotgun in Colorado and a rifle in Virginia and another shotgun in Florida, and the ATF had those serial numbers, that would make me pretty suspicious. If all they had was a mostly accurate list of guns he had purchased in your state I'm guessing they made some phone calls.