Federal agencies can use the “national security” exception to avoid search warrants and use unobtrusive stingrays and dirtboxes to obtain cell phone numbers, owners’ names, and owners’ addresses. Without targeting a suspect, they can access large swaths of data by using this technology in crowds. They can even use that data after participants leave a gathering to track and investigate the people in attendance.
What are Stingrays and Dirtboxes?
https://theintercept.com/2020/07/31/protests-surveillance-stingrays-dirtboxes-phone-tracking/