When did this start to become a thing? Why is it even a thing? Is it just part of the progression of more intrusiveness, or is there something specifically they are looking for? Does anyone know what they do once they have your phone and password? I read some article saying that they looked through your email. Could you just keep one email account on it and delete apps such as facebook, and gmail for instance?
I guess you just wipe your phone prior to travelling and keep a couple of notes on it and do a restore from the internet or icloud or such once you have arrived, presumably they can't compel you to start a 24 hour restore of your phone from an unspecified place.
edit: in retrospect I think phone OS's need to provide as many user accounts as you want under the same name. Enter a different password, get a different profile.
There is something specifically that they're looking for, but it's not on your device. It's your reaction. It's your non-compliance as an excuse to discriminate against you. They can't actually admit that they're discriminating against dark skinned people, or people they overheard making thoughtcrime comments while in line, but they can make intrusive demands and then bust your ass when you don't comply quickly or enthusiastically enough. "Papers please" was never really about the papers.
I guess it does create a situation, and in that situation, as you say, there's opportunity to observe reactions etc. Just seems such a miserable thing, people thumbing through your phone.
It's not practical to not have devices. The only defense against loss of information is to make the information itself useless: bake strong expiration dates and revocable keys, etc. into everything. It should reach the point where losing everything you have to "the shadows" might only harm you for the next 60 seconds until all your keys change.
Just say that you're scared of roaming charges so you left it at home. Just make sure there's some truth to that statement because lying to a border guard is a very bad idea.
https://news.ycombinator.com/item?id=13659864 (115 comments)