When I owned and ran a high tech contract machining company in the 90s, our big brag and marketing hook was that we were ALWAYS ready for a customer visit 24/7. We made it clear that they could visit any time and see anything that pertained to them or the products we made for them, but they would get whoever was free on the shift at the time of the visit as the guide/escort. Sometimes it would be the "ghost tender" who watched over the highly automated shop when it ran "lights out." The ghost tender would tend to a machine when a sensor set out an alert that it was in need (more raw stock, worn tool bit, job complete, etc.)
Everybody in our shop, from the CEO down to the janitor could operate the computer to display records. The records were coded so ONLY records pertinent to a single customer would display, but there was really no reason for them to come to the shop to look at records, since they were available in real time (at first through a direct dial up, then over a virtual private network.) We had one customer whose own shop was about a mile away from ours who came frequently on the ghost shift (about 5 am), bringing doughnuts for our break room. Turned out we were a "port in a storm" since his own place didn't open until 6 am.
Mostly, though, because we made our claim, very few took us up on it over a ten year span, reasoning that we were not causing any other hiccups with quality or delivery, so we didn't need oversight.
Because we WERE making aerospace parts for some customers, we were aware FAA could drop in on us at any time. The local FAA office was less than 10 minutes away and NEVER visited. We were directly FDA registered and similarly never experienced an FDA visit in ten years even though their local office was less than 20 minutes away. My guess is that only "squeaky wheels" (some other problems of quality or delivery) trigger visits from folks (regulators and customers) who do NOT get extra money for visiting. The fact that the unannounced visits we are discussing in this thread are done by third party auditors who generate income with each visit makes me very suspicious of ulterior motives. If I were a small shop where a visit would barely justify 2 or 3 man hours of billable time, I'd worry less about drop-ins than if I were a large shop which would generate 2 or 3 man days of billing.