There was recently a story of a married couple flying. The wife thought the husband was cheating, so after he fell asleep, she unlocked his phone right there and read everything. She didn't handle the news well.
Either way if you're paranoid about security you should use a passcode on top to boot.
Eyes closed, or even eyes looking away, and it just doesn't unlock in my experience.
The best advice if you think you're about to be stopped and have a digital device confiscated is to shut it down, requiring a passcode to open.
There was recently a story of a married couple flying. The wife thought the husband was cheating, so after he fell asleep, she unlocked his phone right there and read everything. She didn't handle the news well.