"They" will show up in a large black van with dish antennas on top of it. First, "they" will act as if "they" are simply canvassing the neighbohood for a yard maintenance company, but you can tell who "they" are by the dark sunglasses. Soon, you will see "them" lurking around, spying, watching, peeking around phone poles, but ONE NIGHT, "they" will raid your home with 1,000 ATF agents, all dressed in black, with tinfoil badges and ID cards that just say "John" hanging around their necks . . .
Seriously, I have never known the ATF to just "show up" and audit a C&R FFL. Heck, I have only met the ATF folks a few times in 25 years.