Portable computing devices have transformed mobility into a serious concern to network researchers. Existing mobility measurement data have been very useful to the community, but achieve their limitations because they apply only to specific contexts (e.g. university campuses and conference venues). This paper advocates using Wi-Fi sniffing to study mobility in other environments. This approach is interesting because of its non-intrusiveness: it does not require access to any networking infrastructure nor active participation from users. This is a realistic approach due to the recent proliferation of Wi-Fi devices. This paper also suggests techniques to extract mobility data from raw sniffer traces.