CBC reporter Kate Dubinski spent time with the London Police Service's human trafficking unit in December. This is the second story in Kate's series, Knock at the Door.

The female officer remembers the 15-year-old well. The girl had no shoes when she was walked out of the place where she was being pimped.

"She was 80 lbs, this tiny thing. We brought her to the station and she was puking and puking because they'd hooked her on drugs and she was in withdrawal," says the officer.

But within hours, the teen was back in the hands of her two pimps, because there was nowhere for her to go. She was too young to be placed in a shelter.

"She hugged me and said 'thank you' when I dropped her back off with this guy and girl who were trafficking her, and I thought, 'I didn't do anything for you.' There were no other options and she wanted to go back."

That call was five years ago, when the officer was a constable on the street.

Today, she's an undercover officer with the London Police Service's human trafficking unit. CBC News has agreed not to reveal her identity.

Five years later, there are still no resources for the youngest victims.

Shelters won't take minors

"Last year I had a 14 year old, my first major case. These girls come to you, they trust you, you want to provide them with every opportunity you can to get out of this, and they want the help, and there's no where you can go."

Most shelters don't take anyone under 16, the officer says.

It's frustrating, and I feel there's a small window where you have the girls ... - Undercover officer

And even then, a 16-year-old victim of human trafficking, sold daily for sex in hotels along Hwy. 401, has different needs than a homeless person or a domestic violence survivor.

Any girl under 16 will be returned to their family or group home. They often don't stay.

The spotty, limited services that do exist have months-long waiting lists because there's so much demand for them.

"It's frustrating, and I feel there's a small window where you have the girls, you have their trust, and they want the help. If you don't get the help now, you'll never get them help."

Hotels just off Highway 401 in London are one of the places members of the human trafficking section look for girls and women being sold for sex against their will. (Colin Butler/CBC News)

Human trafficking is big business in London. Proximity to Hwy. 401 makes the city an ideal spot for a stop along the Windsor to Toronto corridor, with women and girls sold in hotel rooms located just off the highway.

Pimps will earn a girl or woman's trust, and then eventually force her to sell sex.

"It's so predictable. Every girl has the same story. I met this guy, he promised her this, took her money, the next guy did the same thing. There's this hope in them that there's going to be something different with this guy, and there never is," the officer says.

To find victims of human trafficking, officers use two techniques: vice probes, or door knocks, and John stings.

During vice probes, they look for escort ads online and pose as Johns with a woman or girl if they suspect she's being trafficked.

It's not illegal to sell sex in Canada, but it is illegal to buy it.

Det. Mike Hay studies floor plans of local hotels to make sure officers cover all exits and stairwells while checking on a woman they suspect is being sex trafficked. On Friday he pleaded guilty to discreditable conduct. (Kate Dubinski/CBC News)

Posing as a sex worker

During John stings, the female officer poses as a sex worker and men who respond to the ad are charged, a way to put a dent in the demand for sex work.

"Answering the door is always interesting. It's interesting the guys who come," the officer says.

"Some are 18, some are 60, and everything in between. You could be my grandpa, and you're expecting me be to a 20 year old. It's interesting and rewarding to know that they aren't getting away with it and hopefully will think twice about it," she says.

'It breaks my heart'

What surprises her is that people in London, despite news reports, still don't think human trafficking is happening here.

"It's so normalized for us, and people look at us and ask, 'Yeah, but it's not happening here in London, right?' and I just think they have no clue."

"We come across a lot of girls who are independent (sex workers) now but (they tell us), when they were 15, they were pimped out until they were 19," the officer says.

"It breaks my heart when I can't get them the help they need. Talking to my coworkers, and seeing I'm not the only one who feels that way and who is frustrated, that helps."