HOUSTON -

The 2014 Offshore Technology Conference, an enormous trade show, is expected to add 100,000 people to Houston this week.

The annual event also attracts an unsavory element, according to the non-profit group "Free the Captives."

"Anytime you have a large event like 'the OTC,' that draws in tens of thousands of people, sex trafficking increases," said Julie Waters, founder of the anti-human trafficking group.

More than a dozen Houston Craigslist ads appear advertising discreet sexual services for OTC attendees.

"Are you in town for OTC and looking for some fun?...Give me a try... please be disease free," one ad read.

"There are people who tour around the country, following the big conferences, offering these kind of services. There is no doubt," said Mark Stephens, 1991 Houston Police Officer of the Year who is now retired.

Without mentioning specific hotels, Stephens said there is little doubt that some prostitutes are working the lobbies and bars of big hotels this week.

"It's fairly discreet if you know what you're looking for. You can probably pick them out," Stephens said.

Stephens said undoubtedly the Houston Police Department has several secret operations underway in an attempt to ensnare prostitutes and their clients.

The Houston Police Department, perhaps understandably, does not comment about in-progress undercover operations.