Acoustic Triangulation

The indoor perimeter was completely mapped. I slipped through the lower window sash and stepped out onto the cold black iron grating of the fire escape. Above me, the city dusk thickened. A discarded metal soda tab lay wedged between the iron slats near the corner rail. I wedged the rim of my small grappling launcher under its curved ring, levering it free until it snapped into my grip. Slotting the flat aluminum edge into the narrow gap of the exterior brickwork, I struck the surface once with the butt of my tool. A clear, high-frequency ping resonated through the railing, cutting through the low rumble of New York traffic. Distance to the street below confirmed. The rooftop above remains silent.