Within this research-driven project, passive WiFi monitoring of WiFi enabled devices was used to detect users (students, employees, visitors) of buildings at the campus of Delft University of Technology to gain insight into the Rhythm of the Campus: the occupation, duration of stay and moving pattern at and between the different facilities (faculties, library, auditorium). The research was carried out in request of the University's department of Facility Management and Real Estate (FMRE). In total, 20 WiFi monitors were located at the library, the auditorium and five faculties for one week. Simultaneously, ground truth data was manually collected for two days at the library and the auditorium, using an online questionnaire and a counting application. The retrieved data was classified and filtered, which facilitated data analyses and validation. Resulting from statistical analyses a relation between the number of detected mobile devices and the amount of people inside the buildings was developed. This semi-automated method made it possible to get new insights in building use compared to conventional manual counting methods. For visualisation, an online dashboard was developed as a tool to help the client at the decision making process. The research presented in this paper was carried out as a proof of concept. The results presented in this paper give a concise overview of the extensive possibilities of passive WiFi monitoring for facility management