Ways I can think of to identify if someone is/has been in a crowd (in rough order of difficulty): 1. Cell phone data from bluetooth [1] 2. Cell phone data fro

Ways I can think of to identify if someone is/has been in a crowd (in rough order of difficulty): 1. Cell phone data from bluetooth [1] 2. Cell phone data from your carrier [2] 3. Cell phone data from Google/Apple/etc [3] 4. Face recognition with automated methods 5. Face recognition with human super recognizers [4] 6. Gait recognition 7. Using WIFI to detect body shape [5] 8. Using lasers to detect your heartbeat [6] So once technology gets advanced enough, you'll have to wear something and take measures to protect yourself from potentially all of the above if you want true privacy... which would ironically immediately single you out :) [1] https://www.nytimes.com/interactive/2019/06/14/opinion/bluetooth-wireless-tracking-privacy.htm https://www.nytimes.com/interactive/2019/06/14/opinion/bluet... [2] https://www.theverge.com/2019/2/6/18214667/att-t-mobile-sprint-location-tracking-data-bounty-hunters https://www.theverge.com/2019/2/6/18214667/att-t-mobile-spri... [3] https://www.nytimes.com/2019/09/20/us/data-privacy-fbi.html https://www.nytimes.com/2019/09/20/us/data-privacy-fbi.html [4] https://en.wikipedia.org/wiki/Super_recogniser https://en.wikipedia.org/wiki/Super_recogniser [5] https://www.schneier.com/blog/archives/2016/08/using_wi-fi_sig.html https://www.schneier.com/blog/archives/2016/08/using_wi-fi_s... [6] https://www.engadget.com/2019/06/27/the-pentagon-has-a-laser-that-identifies-people-by-their-heartbe/ https://www.engadget.com/2019/06/27/the-pentagon-has-a-laser...