While indoor localization techniques based on Wi-Fi RSS measurements have been extensively studied, their application to eavesdropping Wi-Fi probe requests sent from mobile devices in large indoor environments, such as shopping malls, is scarce or absent in the literature. The idea behind this work is to observe Wi-Fi enabled smartphones, especially when they are not associated to a network. They periodically perform active network scanning by issuing probe requests, which are detected by networked sniffing devices produced by Cloud4Wi®. We experimentally investigate the opportunities offered by passive gathering of Wi-Fi probes for purposes of crowd positioning in areas of interest. Our preliminary experimental setting convincingly shows that a small number of sniffing devices may be enough for analysing crowd movements in indoor areas.