Customers certainly have been arrested and charged as "found-ins" in other cases, without being naked or caught in the act of doing anything except being unlucky enough to be present when the arrests are made.
A raid will occur only after police have already investigated, usually over a month or more, to gather enough evidence to demonstrate that the place has been regularly used for prostitution. The police do not conduct the raid with the hope or expectation of catching anyone naked in order to prove their case. The purpose of the raid is to arrest and charge anyone there, and to also gather physical evidence (cash, condoms, etc.) to further back their case, and they also usually search for illegal drugs.
Charges against the escorts ("inmates") and customers ("found-ins") can often be dropped, sometimes if they agree to provide evidence to help convict the ones in charge of "keeping a common bawdy house".
Edit: This article describes a raid of a massage parlour and some of the investigation leading up to it.
http://www.barrieadvance.com/barrieadvance/article/119988
...Everybody inside gets arrested...