Wireless transmissions from beacon devices are received at a plurality of receiver devices. The wireless transmissions comprise packets that carry information used for location-based services for mobile wireless devices. Content of one or more fields of the beacon packets received by one or more of the plurality of receiver devices is obtained. Measurement data associated with the transmissions received at the plurality of receiver devices is generated. Locations of the respective beacon devices are computed from the measurement data obtained by the plurality of receiver devices. Information identifying detected beacon devices and their locations are stored to maintain a location history of beacon devices over time locations. Changes from information contained in the location history are detected based on the locations computed for respective beacon devices from the measurement data and content of one or more fields of detected beacon packets.