>Which would be a really cool option: a cop or TSA tries to unlock your phone but gets instead a sandboxed environment which makes them think they actually unlocked the phone.

after a few months of that being popular, TSA will institute a policy where the agent will ask a standard question of "are you using a decoy account or otherwise trying to hide the true contents of your phone" every time they inspect a phone. answer yes, they ask for the real account, answer no, and you committed a felony by lying.

Is it true that lying to a cop or TSA officer, presumably when not under oath, is a felony?

Yes (so long as it's a Federal, and not State, official): https://en.wikipedia.org/wiki/Making_false_statements

What is the definition of "true contents"? The sandboxed environment is a real environment. Its contents are true. Not sure you can easily construct a question that makes you guilty of perjury.

I wouldn't trust my freedom on judges accepting that argument.

But if you refuse to answer, that's a 5th amendment right

Your 5th amendment right not to incriminate yourself of course. But that doesn't mean the TSA have to let you fly. They would just turn you away for not cooperating. Just like they would turn you away if you said they couldn't search your bag because of your 4th amendment right to not be searched without probable cause.

as a European, I don't think we get that protection. In this instance its the first plane home

Android has that via profiles.

https://img.gadgethacks.com/img/16/39/63590794005007/0/andro...

I imagine that not many TSA people would think to navigate there and switch it out of the guest profile. No need to lock it, just switch profiles and leave it unlocked.

That would be nice, something like user accounts. Would be useful not just for this. You have one iPad for the family? You probably don't care about those games for 2 years old when you are using the phone yourself - just switch to your account which doesn't contain these apps!

Unless that's already a thing I'm not aware (now using Android)?

It is already a thing on Android.