Wi-Fi probe requests are defined in the Institute of Electrical and Electronics Engineers (IEEE) 802.11 standard as an active mechanism accelerating the connection process and being repeatedly sent to some devices once per minute on average. They contain the unique MAC address of the device as well as other information, e.g., its type and manufacturer. This paper further pays special attention to the received signal strength indicator (RSSI) value, from which an estimation of the distance can be derived. Locating and positioning people in underground structures is important as complex structures make it difficult for rescue and security forces to direct operations and, in the event of an accident or fire, to evacuate people out of the danger zone as safely and efficiently as possible. Existing systems are being tested for the positioning of the task force as well as for civilian detection via surveillance cameras at the Zentrum am Berg (ZaB). To counteract their limitations, RasPos was developed and tested based on the worldwide increase of smartphone users, which could allow the detection of close to 90% of people. First, the behaviour of the RSSI was investigated, then a modified software was implemented that produced two estimates of the number of people in the vicinity. The first estimate includes only the explicit identified mobile devices, while the second estimate represents the number of all Wi-Fi-enabled devices.