We present FALCON, a novel mobile sensing system to approach, localize, and track RF targets via drone networks. We leverage existing Wi-Fi technology and its recent Fine Time Measurement (FTM) protocol to realize the first FTM sensing drones that can dynamically range targets in a mission. FALCON is also the first robotic system realizing FTM for autonomous navigation. In addition, we propose a new flight planning strategy to simultaneously approach and localize a target to enable higher resolution sensory measurements and to realize approaching-critical tasks in a mission in addition to localization and tracking. For that, we propose to jointly exploit drones' diversity of observation and dynamics of approaching the target, and dynamically adjust the intensities of approaching and observation based on a mission requirement. Our implementation of the flight planning strategy on custom FTM-enabled drones shows that FALCON achieves up to 2x localization accuracy gain compared to a baseline Bio-inspired approach, and in total spends 30% less time in localizing the target.