A flight attendant notices a teenage girl uncomfortable with the older man she’s traveling with and leaves her a note in a bathroom. The teen writes back: “I need help.” A teenage boy from New Zealand on a one-way ticket to visit a sex offender is intercepted by a careful Los Angeles customs agent.

These are two stark examples of how airlines, airports and the Department of Homeland Security are stepping up efforts to thwart human trafficking, which is transporting people for forced labor or commercial sexual exploitation....