The Out-of-Home (OOH) media markets currently rely on a plethora of methods and technologies for measuring their potential or real success. From nationwide interviews over statistical analyses to device based mobility records, each of the OOH markets (in different regions or countries) has chosen its own mixture of measurement efforts in order to provide more or less clear numbers to their clients. In this contribution, we are presenting the Presence Scanner, a new and inexpensive technological approach to detecting the presence of people in front of OOH media sites. Based on an array of ultra sonic range finders and equipped with wireless communication capabilities, the Presence Scanner is on its way to resemble a flexible and scalable solution to plug and play activity sensing in (at least) indoor scenarios.