Power wastage in dense WLANs is a challenging problem. In order to reduce the power wastage, AP switching on/off strategies have been developed, aiming at decreasing the number of active APs (according to the traffic conditions). The efficiency of such strategies is strongly dependent on the efficiency of detecting WLAN users communication attempts, as a trigger for switching on the inactive APs. We demonstrate that such a communication attempts can be efficiently identified even for SNR values far below thresholds needed for frame reception. We present a signal-detection-based approach and verify its efficiency experimentally.