This is why it’s so great that iOS 11’s new easily-invoked Emergency SOS mode requires you to enter your passcode after invoking it. When you’re entering customs or in a situation where you’re worried you’re about to be arrested, you can quickly disable Touch ID without even taking your phone out of your pocket.
Until iOS 11 ships, it’s worth remembering that you’ve always been able to require your iPhone’s passcode to unlock it by powering it off. A freshly powered-on iPhone always requires the passcode to unlock.
This unfortunately does not help at borders, which you should take into account while traveling to countries such as Russia, China, USA, and Australia, amongst others:
In fact, US Customs and Border Protection has long considered US borders and airports a kind of loophole in the Constitution’s Fourth Amendment protections, one that allows them wide latitude to detain travelers and search their devices. For years, they’ve used that opportunity to hold border-crossers on the slightest suspicion, and demand access to their computers and phones with little formal cause or oversight.
Even citizens are far from immune. CBP detainees from journalists to filmmakers to security researchers have all had their devices taken out of their hands by agents.