The analysis of people mobility in urban contexts is of key importance to tackle major issues in different fields, such as those related to urban planning, citizen safety, telecommunications services planning, public transport service deployment. Such an analysis should provide key indicators, such as crowd density per area of interest, people flows, recurrent mobility patterns, mobility heat maps, just to cite a few. In the present paper, we provide a solution named PmA, which relies on the monitoring of WiFi traffic, which is also solicited through the generation of probe requests. The main features of the solution are the followings: preservation of user privacy, extraction of key metrics on user return and permanence, computation of mobility heat maps. Extensive sessions of monitoring and analysis have been carried out in three different pilots: an university campus during classes, an international fair, and a roundabout.