Don't they make regular visits to all such establishments?
They "turn a blind eye" to the fact that they are all technically brothels but do periodic checks to make sure the girls are safe, ie no aggressive punters, and that the girls are legit, ie not trafficked.
Additionally, they will visit if there's a report from a punter which could either be due to a genuine concern that a girl(s) is being coerced or a disgruntled punter who reports them maliciously.