"The Marauder's Map" is a magical map in J. K. Rowling's fantasy series, "Harry Potter and the Prisoner of Azkaban". It shows all moving objects within the "Hogwarts School of Witchcraft and Wizardry". This paper introduces a similar attack to location privacy in wireless networks. Our system, namely the digital Marauder's map, can reveal the locations of WiFi-enabled mobile devices within the coverage area of a single 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 and instantly used 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.