Bluetooth devices naturally emit many public signals. It opens new paths for passive mobility analysis, and allows building larger datasets direly needed by the research on mobile systems, but also raises new practical challenges. One of them is the correlation between the public packets and the emitters. The Bluetooth standard forces devices to regularly change the identifier they carry in the public packets, but we show in this paper it does not prevent from correlating packets from a given sender. The number of devices changing their MAC address at any given time is expectedly small; we only need to find a property differentiating a handful device at a time for MAC association. In this paper, we propose such a property and demonstrate the efficiency of the association strategy on Bluetooth Low Energy.