A friend of mine had it happen to him. He got a call and made an appointment with them when it was convenient for both of them. He said the lady who showed up was very polite and professional about it. She looked through his bound book and asked to see a few firearms at random, which he held for her with his white cotton gloves on while she verified the serial numbers (very collectible Lugers mostly).
He said the ATF lady was only there maybe 30 minutes and never said what triggered the visit.
Disclaimers: I am not a lawyer, cop, soldier, gunsmith, politician, plumber, electrician, or a professional practitioner of many of the other things I comment on in this forum.