Several contact-tracing app-based solutions have been proposed to alert app users who have previously encountered a Covid-19-positive user. In most countries, a statesponsored app has been made available to let citizens who install it on their phone monitor inter-personal contacts.The particular scenario considered in this work aims to support tracing within a university campus. For this, an alternative solution is proposed, based on a hybrid decentralized approach, that traces people who have been present in the same environment within some meaningful time interval. Thus, tracing goes beyond direct contact between persons. Our solution can also monitor crowd gathering on campus premises.The proposed Android app senses surrounding WiFi signals and uses them to obtain the user's absolute location. The app then securely sends an anonymized presence data object to the server. Thanks to data thus gathered, as soon as a user has reported herself as Covid-19-positive, all apps will be alerted by the server and receive anonymous data to determine whether their users happened to be in the same environment as the Covid-positive one.We believe our approach to be both effective, for it eschews weaknesses and limitations of Bluetooth-based solutions, and viable, for the experiments reported have proved WiFi sensingbased localization to be accurate enough.

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The authors acknowledge the support provided by project TEAMS -TEchniques to support the Analysis of big data in Medicine, energy and Structures -Piano di incentivi per la ricerca di Ateneo 2020/2022.