Since the beegining of the Covid-19 crisis, states and private company work on App to track the people who can be infected because they were more than 15minutes at less than 2 meters in the last 2 weeks close to someone with a positive test.
They are using Bluetooth to communicate between the devices which “comply” to the UE policy but …
You have here 2 documents one in French simple and easy to read that explain simply how it work and clear privacy/security issue. The next document is about SwissCovid and issues this article is a real scientific publication a bit moure complex to understand.
French : https://risques-tracage.fr/docs/risques-tracage.pdf
English : https://gitlab.com/PersonalDataIO/swisscovid-risk/-/raw/master/swisscovid-risk.pdf