With the abundance of ubiquitous transmission of wireless signals nowadays due to the proliferation of wireless devices, these man-made radio frequency signals in the form of electromagnetic waves can be exploited in various forms, for example localization and tracking. Here, we describe how these signals can be used in sensing an object or human in the environment. We describe the design concept and implementation of an indoor passive tracking system that utilises an array of Wi-Fi transceivers, and without any electronic device or tag attached to the object being tracked. The sensing of an object or a person described here is made possible by exploiting the fundamental characteristic of signal attenuation due to blocking, i.e. shadowing, that is prevalent in a typical wireless communication system. By detecting significant signal attenuation in the system (i.e. by measuring the received signal strength value), it is possible to infer that an object is blocking the line-of-sight (LOS) link in a transceiver set and therefore transforming the existing hardware configuration into a proximity sensor network.