Wireless sensor networks (WSNs) are composed of autonomous sensor nodes that cooperate among themselves to monitor scenarios in environmental, industrial, civilian and military applications. The wide applicability of such systems stimulates the growth of intelligent services to make living environments more comfortable and safer. Concerning security and surveillance issues, WSNs have been widely integrated with multi-modal sensors and actuators in order to actively detect any abnormal events. Though the sensor fusion provides accuracy and robustness, problems such as high costs and consumptions exist. In order to reduce the hardware complexity, other systems for detection and tracking of human activities interact with active devices worn by the targets. In this work, a WSN-based solution is proposed for the detection of transceiver-free targets exploiting only the information provided by the wireless links among the nodes.