1. A device for tracking service queues, comprising: a first antenna configured to receive wireless signals from a plurality of mobile wireless devices, each wireless device representing a subject entering, being serviced in or leaving a service queue; a receiving circuit coupled to the first antenna and configured to measure a signal strength amplitude of the received wireless signals from the first antenna; a processing device communicatively coupled to the receiving circuit; and a non-transitory computer readable medium in communication with the processing device, the computer readable medium storing one or more programming instructions for causing the processing device to: receive a trace comprising a plurality of received wireless signal strength amplitudes from a selected one of the plurality of mobile wireless devices in a temporal order from a time the selected mobile wireless device enters a receivable range of the first antenna to a time the selected mobile wireless device exits the receivable range, determine a slope of the trace, identify first segments of the trace containing a mostly positive sloped received signal strength amplitude, identify second segments of the trace containing mostly negative sloped received signal strength amplitude, extract attributes from the first and second segments, and input the extracted attributes in a Bayesian Network model to compute a beginning of service and a leaving period of the service queue, wherein the Bayesian Network model comprises two parent nodes, each respectively modeling a beginning of service and a leaving period of a service queue; wherein the attributes comprise R, S, L LP , C LP , L BoS , and C BoS , wherein: R=1 when a mean RSS before a specific segment of continuous negative slopes is the maximum of all segments of the trace, otherwise R=0, S=1 when a variation of RSS within a window W before a segment of continuous negative slopes is smaller than a threshold, otherwise S=0, L LP =1 when a starting time of a specific segment of continuous negative slopes is later than a time where a maximum RSS value occurs, otherwise LLP=0, C LP =1 when an average slope of a specific segment of continuous negative slopes is the minimum of all segments of the trace, otherwise C LP =0, L BoS =1 when an end time of a specific segment of continuous positive slopes is earlier than a time where a maximum RSS value occurs, otherwise L BoS =0, and C BoS =1 when an average slope of a specific segment of continuous positive slopes is the maximum of all segments of the trace, otherwise C BoS =0.