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Don’t ever hand your phone to the cops
(www.theverge.com)
This is a most excellent place for technology news and articles.
Like taking your phone and go through your WhatsApp messages?
If that's a concern you could set up a password to access any sensible app or chat within that app.
I think that is a more sensible approach. As if you are targeted by any reason an undercover cop could get a hold on your unlocked phone by many different ways.
Seems like a good use for android app pinning, I think that locks the phone to that app until unlocked