In the final part of 'Rape on the Night Shift,' we learn about a team of unconventional investigators working to hold the janitorial industry accountable.

The lights go out as the daytime office workers leave the building. And then, room by room, they flicker back on -- three stories of glass, lighting up like a shadow play. The silhouettes of janitors appear -- one polishing a window, another mopping.

Two women are hiding outside, behind a palm tree, watching.

“Look, do you see them?” says Veronica Alvarado. “There’s a guy walking, he’s passing the vacuum, on the second floor.”

She and Vicky Marquez are casing this office park in suburban Orange County. Marquez is less than 5 feet tall with heels on. Alvarado has tattoos and green highlights streaking through her hair. Hardly the picture of undercover investigators.

“So, we’re walking around the building to see the possible entryways. The workers are probably parking in the back of the building,” says Alvarado.

They’re scouting out dumpsters where they might meet janitors taking out the trash. They want to talk to them about their working conditions.

Marquez worked as a janitor for 15 years. She says, many times, janitors are getting work through unscrupulous subcontractors.

“Nobody can trick me and say some of this stuff is legal,” says Marquez in Spanish. “When they say they’re getting paid cash, their check doesn’t have a stub, or they tell me they earn $500 or $600 every two weeks, working six or seven days a week.”

Marquez and Alvarado work for a nonprofit that’s trying to root out abuses in the janitorial industry. The Maintenance Cooperation Trust Fund, or MCTF, deploys teams of investigators across California. It’s a joint effort by the Service Employees International Union, representing janitors, and unionized janitorial companies, who don’t want to be undercut by black-market competitors.

On this night, the investigators are racing to get inside an office building.

“Don’t run!” says Marquez. “That makes us look suspicious to the security cameras.”

“Magic, magic!” yelps Alvarado. They find the door still unlocked and head to the second floor, where they introduce themselves to two janitors and start talking to them about their working conditions.

On this night, they find some janitors who tell them they work seven nights a week without overtime, others who say they have to buy their own cleaning supplies.

But once in a while, they meet a woman who confides a darker secret, that she’s being sexually abused on the job. Georgina Hernandez was one of them.

“When you need the job, you become a victim by not having the courage to say ‘no,’ " says Hernandez in Spanish. “And if you say ‘no,’ you are going to lose the job. I didn’t have anyone to tell, someone to trust.”

Hernandez met investigator Vicky Marquez while working for a cleaning company that didn’t pay overtime or give rest breaks. The watchdog group helped Hernandez and other workers file a complaint with the state labor commissioner.

Hernandez grew to trust Marquez, who says she confided that a supervisor at another job sexually assaulted her.

“She said there were times when she wanted to die,” recalls Marquez. “As a woman, it’s hard to stand seeing another woman feeling like that.”

“If I hadn’t met Vicky, I would have kept all this pain inside,” says Hernandez. “I wouldn’t have known how to speak about what happened.”

Eventually, Hernandez filed a civil suit, which settled last year. She can’t talk about the details of the case because she signed a confidentiality agreement with the janitorial company. But she could tell us how the experience changed her.

“I have really learned not to let anyone take advantage of me, to say 'no,' ” says Hernandez. “I have learned to tell the other women I work with not to let anyone take advantage of them. That there is help for us, as janitors.”

Hernandez is just one of the women we met during an 18-month investigation that exposed the problem of sexual abuse at janitorial companies across the nation. The industry is full of shadowy subcontractors. Even established companies with policies to protect workers have bungled investigations and have hidden complaints through secret legal settlements. Government agencies charged with worker safety have largely ignored the issue.

As for Georgina Hernandez, she wonders why cleaning companies don’t ask janitors what they think would put a stop to sexual violence on the job. How about a buddy system for workers, or more women supervisors? Whatever it takes to protect janitors on the night shift.

Our series “Rape on the Night Shift” was reported with Bernice Yeung and Daffodil Altan at Reveal from the Center for Investigative Reporting, and Lowell Bergman and Andres Cediel at the UC Berkeley Investigative Reporting Program. It was produced in collaboration with Frontline and Univision.