Does he? I don't remember a scene displaying such a skill, plus Bill also said he doesn't remember telepathic communication between Jinwoo and the shadows. He does know when one is destroyed, though. Perhaps a hiding shadow was accidentally destroyed by the humanoid ant, triggering Jinwoo's sense of crisis immediately. Other than that, can his shadow soldiers hide anymore in the shadow of a dead person?
They can look for anything strange and then either deal with it or come back to report to him. He can also switch places with them when convenient.
Back in the army, patrols were sent out, even though the commanding officer obviously couldn't see what the patrols could see. The patrolling soldiers were assumed to react appropriately when noticing anything strange or alarming.