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Wi-fi enabled devices such as today’s smart-phones are regularly in-search for connectivity. They continuously send management frames called Probe Requests searching for previously accessed networks. These frames contain the sender’s MAC addresses in clear text, which can be used as an identifier for that sender. Being able to sniff that MAC address at several locations allows us to understand the mobility behavior of that device. In this paper, we present a solar-powered, beagle-bone based standalone system that continuously sniffs the air for probes and extract their MAC addresses. We deployed the system in the world’s largest gathering (The Hajj) and tested it at scale. Our objective was to build an infrastructure for non-invasive mass crowd analysis. Our deployment had a total of 8 sniffers covering a population of 185,000 people. We detected 37.5% of the population, analysed their arrival and departure behaviours, identified their smartphone manufacturers and extracted their transition patterns from one sub-location to another. By presenting valuable insights on the mobility of our target crowd, we validated the potential of our platform for crowd mobility analysis.