We present Wi-Peep – a new location-revealing privacy attack on non-cooperative Wi-Fi devices. Wi-Peep exploits loopholes in the 802.11 protocol to elicit responses from Wi-Fi devices on a network that we do not have access to. It then uses a novel time-of-flight measurement scheme to locate these devices. Wi-Peep works without any hardware or software modifications on target devices and without requiring access to the physical space that they are deployed in. Therefore, a pedestrian or a drone that carries a Wi-Peep device can estimate the location of every Wi-Fi device in a building. Our Wi-Peep design costs $20 and weighs less than 10 g. We deploy it on a lightweight drone and show that a drone flying over a house can estimate the location of Wi-Fi devices across multiple floors to meter-level accuracy. Finally, we investigate different mitigation techniques to secure future Wi-Fi devices against such attacks.