A recent proliferation of mobile devices led to immense deployment of WLAN access points (APs). IEEE 802.11 mobile devices periodically perform active background scanning to discover available APs in the vicinity. However, this periodic active scanning imposes unnecessary overhead in a static WLAN environment. In this paper, we address this scanning overhead issue of mobile devices in IEEE 802.11 WLANs and propose a smart scanning method using a motion sensor. Our scheme scans channels only if the signal strength from the associated AP becomes below a specified threshold and a mobile device is moving. The smart scanning method suppresses unnecessary periodic background scans. We implemented the smart scanning in a commercial smartphone and evaluated its performance in a real indoor WLAN environment. The experimental results show that the smart scanning can significantly reduce the scanning overhead, the number of unnecessary handoffs and force disconnections.