Femtocells are small cellular telecommunication base stations that provide improved cellular coverage. These devices provide important improvements in coverage, battery life and throughput, they also present security challenges. We identify a problem which has not been identified in previous studies of femtocell security: rogue owners of femtocells can secretly monitor third-party mobile devices by using the femtocell's access control features. We present traffic analysis of real femtocell traces are presented and demonstrate the ability to monitor mobile devices through classification of the femtocell's encrypted backhaul traffic. We also consider the femtocell's power usage and status LEDs as other side channels that provide information on the femtocell's operation. We conclude by presenting suitable solutions to overcome this problem.