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Stop Using Your Face or Thumb to Unlock Your Phone
(gizmodo.com)
This is a most excellent place for technology news and articles.
The concern with bringing your phone is that police have subpoenaed cell providers to force them to turn over cell tower records. The police then used the lists of cell phones connected to those towers to track down protestors.
You shouldn’t bring your phone to a protest because it could end with police kicking your front door in three weeks after the protest has wrapped up.