In the recent years a wide range of sensors has been developed in the field of human presence detection for user-centered applications. Some of them are not focused on detecting people themselves but the electronic gadgets that they use to carry, which are basically mobile phones. On this basis, and taking into account that almost everyone owns a cellular phone nowadays, the overall goal of this thesis is to explore a new way to passively detect persons by means of their mobile phone emissions. Modern phones make use of several technologies, most of them valid for the realization of sensors. However, this thesis will be exclusively focused on the Global System for Mobile Communications (GSM) that has been used for cellular telephony from the early nineties to the present day. The work relies on the literature as a starting point for a complete understanding of the intricacies of the second-generation digital cellular networks. A method for detecting nearby mobile phones that are not currently being used is proposed. The method is implemented as a proof of concept using a Software Defined Radio (SDR) peripheral together with the proper open-source software to test the feasibility of the whole idea. While the detection of inactive mobile phones could not be shown using the proposed method, the reliable detection of actively used phones has proven to be possible. As the initial approach could not be implemented successfully, the aforementioned sensor will be limited to the detection of active mobile phone users.