Which method is NOT an active search technique?

Searching from a helicopter is not considered an active search technique because it primarily involves a passive observation from an aerial viewpoint, rather than actively engaging in search efforts. Active search techniques usually require hands-on participation and direct involvement in examining the search area, which often includes methods where rescuers physically interact with the environment.

Probing under water, human chains, and the use of sonar or thermal imaging all represent methods where searchers are actively trying to locate a submerged person or object. Probing involves physically using tools to search the underwater environment, while human chains utilize teams of individuals working together to systematically cover an area. Sonar and thermal imaging can also be classified as active techniques when they are employed in conjunction with physical search efforts, as they are tools that assist searchers actively engaging with a scene.