We've had a couple of visits this year across various plants, sometimes they're kind enough to call us when they're headed up the street to the plant. Almost all visits (so far as I'm aware) will be part of an investigation, either into your company or a product you're related to. In our case, they wanted to sample and test cantaloupe as part of their ongoing look into the 2022 outbreak. We weren't involved in any cases related to the outbreak they tracked, but now they're heavily scrutinizing cantaloupe all over just to be safe.
They'll usually bring a team, and they'll invite themselves to everything from your production lines, storage areas, employee interviews, records, you name it. Of course, since they're already there, you can expect them to conduct an audit. And if you're lucky, you'll never hear from them again as they're not great about sharing findings unless they find something wrong.