As a former FFL, LE has access to all firearm transactions @ the wholesale level. When they want the retail answer they typically call the FFL from whence their interest lie. If BATF visits, you open the books, but you don't get to proctor their inquiry. Got rid of my FFL before the agencies went noticeably rogue. Out of business? BATF gets your folder of 4473s.
Got a call from the FBI looking for the owner of a Colt Python, 1 of 10000 in the series. The ones I sold were 8000 range. Less than 24 hours after the crime where they recovered the gun. I informed FBI my guy still had his gun, no names.