That reminds me of one of my first tax enquiries back in the early 90s. Our firm had a client whose "books" looked fine, good turnover, good margins, healthy profits, etc., all tax returns and submissions on time. Classic case of a "low risk" business.
That was in the days of routine/random VAT inspections where a local VAT inspector would turn up for a day to look through the books, invoices, bank statements, etc. All perfectly normal - they'd often pick up a few things, send a bill for a few hundred pounds of tax accidentally unpaid due to overclaiming something. We'd often have them in our office if we did the book-keeping, VAT returns, or payroll, as we'd be more help explaining the books and returns than the client.
This particular client, we had no concerns, VAT inspector came in, we gave him an office with the usual boxes of books, invoices, till slips, bank statements, etc., and left him to it.
We only started to worry when it got to 5pm and he said he'd be back the next day, which was pretty unheard of. He'd not spoken to us other than pleasantries, not asked any questions, etc. When he did the same at the end of the second day, we started to worry! The partner asked a couple of us who'd been involved with the books to stay behind after work, and we spent a couple of hours reviewing the files, tax returns, yearly accounts, etc and none of us could spot anything to explain why the VAT man was going to take at least 3 days on the review!
After 5 days, he said he'd finished. Refused to talk to us about what he had or hadn't found. Just said he'd be in touch.
It all went "legal" from then on. Client ended up being sent to prison for tax evasion.
It turned out he'd not just been siphoning off some sales into a different till - he'd have got away with just paying the tax and a fine if he'd done that! What he was doing to make the percentages look right, was also buying stock from different wholesalers and not putting the costs through either (to much cost without sales incomes would bring margins down to arouse suspicion). So, HMRC (Or HMCE/IR as it was then) prosecuted him for complex tax fraud. They'd discovered what he was doing because the VAT man had done a record check of various wholesalers, taking notes of customers, sales invoices dates and amounts etc., and then going round their "customers" cross check the customer business (shop, cafe, etc) was claiming the same expense and the wholesaler was showing as income - basically cross checking one business to another. When he discovered our client hadn't been claiming costs for purchases from this particular wholesaler - not just one invoice that could have been "forgotten", but literally dozens and dozens of invoices not claimed for, he identified what was going on which is a basically common tax fraud of "two sets of books", not just splitting sales, but two completely different sets of ins and outs to stop margins and percentages looking wrong.
Quite fascinating when you see the lengths some people go to when it comes to tax evasion, benefit fraud, etc. It's FAR more than pocketing a bit of cash or putting some shop sales in a separate till/box.