I think the pilgrims on the Valcarlos route alerted officials to the missing cohorts.
"Also the Civil Guard of Traffic was forced to collect in the top of Ibañeta two of them, who presented / displayed leg ailments and symptoms of hypothermia. In the middle of the afternoon, they were taken to the Roncesvalles shelter, where they could rest and recuperate. Indeed, the South Korean Hyeonje was one of the two pilgrims who gave the notice from Roncesvalles that two of his companions had not yet reached their destination."
The sign is at the intersection of the two routes, so the entire group saw it. Part went Valcarlos, part decided on Ibaneta.