In emergency scenarios such as earthquakes, fires, avalanches, or building collapses, it is necessary to discover people trapped under debris or anyway hidden from eyesight. In this paper, we propose RescuePal, an energy-efficient smartphone-based system that does not require any interaction by the victim and does not use energy-expensive GPS. RescuePal leverages a wake-up system based on sounds that activates the WiFi interface of the victim's smartphone only when the rescuer is close, to save energy. After presenting the system, we mathematically formulate an optimization problem so as to find the sound frequency and power level that minimizes WiFi false activations and yet guarantees high discovery efficiency. RescuePal has been implemented on off-the-shelf Android-based devices, and its performance has been evaluated on a realistic use-case scenario of victims inside a building. Finally, the energy consumption of RescuePal has been calculated using the Power Monitor hardware tool. Results demonstrate that RescuePal is highly effective and saves more than 60% of energy with respect to an approach based only on WiFi.