You go in for a massage ... and then she offers to do what?!

You hear about it every once in a while.

Police in some New Jersey town will raid a local spa and arrest women who are charged with offering ‘happy endings’ in addition to any actual massages.

This might sound like it’s no big deal to some, but frequently in the Garden State, these situations involve human trafficking.

According to Rochelle Keyhan, the director of the disruption strategies department at the Polaris Project, an organization focused on eradicating human trafficking, many of the women engaged in prostitution at massage parlors come from other countries, and are held against their will.

“Most of the women trafficked in illicit massage businesses are between 35 and 55 years old and they’re directly from China or Korea. They typically don’t speak English, they don’t understand our social service systems, our language and our culture," Keyhan said.

She said the vast majority of these women get unwittingly snared into prostitution.

“Typically, there’s something in their life that occurred that makes getting money a necessity — a family member will be sick, there’s some urgent need, and that makes them incredibly vulnerable.”

According a recent study by Polaris, there are at least 373 illicit massage businesses operating in New Jersey.

“When you look at what defines human trafficking, it’s forcing someone to engage in the commercial sex or labor, via force, fraud or coercion," she said.

She said typically these women are moved around, rotated from location to location by the organized crime organizations that control the massage parlors where they work.

“They’ll be unaware of exactly what city they’re in at any given time. Sometimes it’s every few weeks, sometimes it’s every few months, every once in a while it’s every few days," Keyhan said.

She added it keeps the women “more disoriented, and allows for more control.”

So how can you tell if a massage parlor is offering a real massage or a sexual service?

She said legitimate spas are designed to make you feel comfortable, with bright open windows that offer a welcoming ambiance.

“These illicit massage businesses typically have blackout tint on their windows with curtains drawn and the door is bolted shut," Keyhan said. "Nothing about it calming or invites a therapeutic energy.”

She said there is also frequently a lot of security. “For example, multiple deadbolts or multiple CCTV, like surveillance cameras pointing down at the door, to see who might be at the door."

She said another tipoff is “you hardly ever see anyone enter, because the clientele typically enters through a door, like a side or a back entrance.”

She said if a woman goes to this type of location and tries to book a massage, she’ll frequently be told there is no availability.

“Or sometimes they’ll just flat out say this place is not for women, only men come here, and they’ll just tell you," she said.

She said sometimes there is no massage offered at all in these operations — but even if there is, “men will be offered a happy ending or the woman will sort of make a move to do it.”

She noted more established businesses may even offer Groupons.

“Part of the tactic there is to present a veneer of legitimacy, to avoid detection of the illicit behavior that’s occurring," Keyhan said.

She encouraged anyone who suspects a massage spa may not be legitimate to call authorities and report it.