If LE does not eye-witness a sexual service in progress, or find evidence of a sexual service having occurred (trash bin, etc...), there is not a reasonable amount of evidence to arrest a client leaving the MP. Is this a solid assumption?
Will police have to send in undercovers as customers, to have a chance of witnessing something going on?
Unfortunately, not. Eyewitness evidence is not limited to LE; it can come from anyone. If direct observation evidence is available, it's very important, but its can be just one piece of the puzzle. And, if you ask defence counsel, eyewitnesses can be quite unreliable.

