At 9:24 ingame time you hear a scan already going off (turn volume loud). You can also see the field of vision near those Lurkers slightly expand (bottom right of the vision circle close in which the Lurkers are).
Flash had just scanned the Lurkers a few seconds before the second on-screen scan that was only for safety so that he doesn't run out of detection mid-combat so to spectators it looked like he just went there, split his forces and then scanned.