1. A computer-executable method for detecting in-person meetings between users, the method comprising: obtaining, by a server, feature information representing characteristics of a surrounding environment detected by two mobile devices associated with the users, and that indicates a location and/or distance between the two mobile devices, wherein the characteristics include Wi-Fi signals detected by the two mobile devices; producing, by the server, input observations to a hidden Markov model from the collected characteristics information, which involves comparing the Wi-Fi signals detected by the two mobile devices to produce the input observation associated with Wi-Fi signals; and determining, by the server, the presence of an in-person meeting between the users, which involves applying the hidden Markov model comprising a plurality of states that each reflect a meeting state, wherein the hidden Markov model includes at least a state indicating that the users are not in a meeting, and a state indicating that the users are in a meeting.