"The Marauder's Map, a magical map in J.K. Rowling's fantasy series Harry Potter and the Prisoner of Azkaban [CHECK END OF SENTENCE], can be used as a surveillance tool to show all moving objects within the boundary of "Hogwarts School of Witchcraft and Wizardry at a spell. In this paper, we introduce a similar forensic surveillance tool for wireless networks. Our system, the digital Marauder's map, can reveal the locations of WiFi-enabled mobile devices within the coverage area of a high-gain antenna. The digital Marauder's map is built solely with off-the-shelf wireless equipments, and features a mobile design that can be quickly deployed to a new location for instant usage without training. We present a comprehensive set of theoretical analysis and experimental results which demonstrate the coverage and localization accuracy of the digital Marauder's map. © 2006 IEEE.