You can only audit whatever is declared. Nightclubs (for example) routinely use their door money to pay DJs and other casual staff cash in hand and off the books, with the remainder finding its way into the pockets of the owners and (sometimes) the management. If that money isn’t rung through (or is rang into a different, unconnected till) then no auditor is going to know (or probably care) about any skim, as long as the protagonists don’t get too greedy and start declaring figures that descend into the realm of the absurd.
A cash turnstile or two would provide the opportunity for an owner of a club to feather their own nest, but also to make certain payments off the books, which might suit the way the wanted to run the business and enable them to skirt round the rules - which some people enjoy doing for its own ends because they are wired that way.