Honolulu, June 24 -- Yesenia D'Alessandro loaded a GPS tracking app on her cellphone and trudged into a remote Hawaii forest, joining more than 100 other volunteers looking for a missing hiker.

She climbed through muddy ravines, crossed streams and faced steep drop-offs in the thick tangle of trees and ferns where her college friend Amanda Eller vanished last month.

"You have to search everywhere," said D'Alessandro, who flew in from Maryland. "You have to go down to that stream bed, even though you don't want to. She could be down there."

D'Alessandro and others gathered GPS data of the ground they covered, and organizers put it on a specialized digital map to help better understand where to look next.

The technology led volunteers t...