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How Do I Prepare My Phone for a Protest?
(themarkup.org)
This is a most excellent place for technology news and articles.
If your phone is powered your location can be tracked. Secured or not: your location can always be tracked.
The rest is doubtful, always assume law enforcement has access at will.
Just don't bring it.
doesn't have to be powered on.
unless you can remove the battery, it should stay at home.
Source? EFF says airplane mode is sufficient per the article