Nowadays, wireless sensor networks are deployed in a wide range of applications such as military. To enable sinks to avoid physical attacks from adversaries, most of WSNs adopt sink-location privacy mechanisms. By utilizing these mechanisms, an adversary cannot analyze packet traffic and perform hop-byhop trace-back, and thus deduce the location of a sink. In this paper, we propose an attack approach to track anonymous sinks. It utilizes a Pseudo-Noise (PN) code to mark a data flow in an invisible manner. An adversary is able to interfere with a source node's traffic by embedding a secure signal into the node's traffic. The signal is carried along with the traffic from the source node to the sink. Therefore, the attacker can recognize the location of a sink node by tracking the invisible secure signal. Through our simulation experiments, we conclude that the proposed attack approach is able to track an anonymous sink without additional traffic overhead.