Harbinger had followed a blue figure, noticing signs of injury. His puppet was unarmed, and had sustained damage which prevented access it it's biotic abilities, so he wasn't going to risk further damage to it by confronting armed individuals, and while the blue dressed man was armed, he was in no condition to fight. While not as powerful as his own sensory tools, his puppet was able to detect that the man he had followed was human, although also something else. He didn't bother speaking, having never had a reason to speak to anyone. Reapers were ancient, and among them, Harbinger was one of the eldest.