Ghost picked up some intel from the Fallen troops you defeated. Phylaks has platoons patrolling Asterion Abyss, trying to claim Vex-held territory for Eramis.
Defeating these platoons will draw out Phylaks's commander, Elenaks. This should lead to more information about her whereabouts.
"Phylaks uses her soldiers like pieces on a game board. You must draw her out. Give her no choice." —Variks the Loyal