Opening your phone with your fingerprint or facial recognition is cool and convenient. But it also means people can easily unlock your phone without permission - while in the process of being robbed say, or if you're arrested on suspicion of committing a crime yourself. Thankfully iOS and Android let you temporarily turn off fingerprint or face recognition with several different methods.
Why would you need to quickly lock up your phone? Well, several courts have ruled that law enforcement can't force you to enter your phone's passcode, but they can force you to open your phone with your fingerprint. This is especially pertinent if you spend any time in the US, where everyone from the TSA and customs to traffic cops and riot squads have the right to snoop through your phone. Fortunately, it's easy to disable instant phone access on Android and iOS. Here's how:
Android P (now in beta) includes a "lockdown mode" that requires your swipe pattern or passcode. Open Settings > Security & location > Lock screen preferences. Toggle on Show lockdown option. Then any time you want to activate lockdown, hold down your power button and tap Enter lockdown.
On Oreo, open Settings > Lock Screen and Security > Screen Lock Type and toggle off all the options under the Biometrics section.
You can make a home screen shortcut for the setting using the Tasker app ($US2.99 ($4) on Google Play). Create a "System lock" task (Display > System lock) and add a shortcut to your home screen. Then whenever you need to lock up your phone, tap the shortcut.
Power and Volume Buttons
The easiest, least-detectable way to turn off Touch ID or Face ID is to hold down the power button and either of the volume buttons. (On iPhones 5S to 7, hit the power button five times.)
After a few seconds, your phone will vibrate and offer three options: Power off, Medical ID, and Emergency SOS. Whatever you select, even Cancel, your phone will ask for your passcode to unlock.
Once you unlock your phone, Touch ID will be enabled again. So you have to repeat this step every time you lock your phone.
Those lock screen options are pretty cool: Medical ID shows whatever emergency contact info you've stored. Emergency SOS immediately calls 911. If you cancel that, it counts down ten seconds before calling your emergency contacts. If you cancel that, it finally calms down. But your phone will still require your passcode to unlock.
If you have Listen for "Hey Siri" enabled, then saying "Hey Siri, whose phone is this?" to your locked phone will disable Touch/Face ID. Like the button-press method, this only lasts until you've unlocked your phone once.
Weirdly, this doesn't work if you activate Siri by holding down the Home button. (Siri transcribes your question as "Who's phone is this?" — an embarrassing grammatical error that will surely crater Apple's stock price.) When we tested it, the voice command sometimes failed unless we tapped our own contact info. This feature is apparently too clever for its own good.
On iOS, you can go into Settings > Touch ID & Passcode (or Face ID & Passcode) and toggle off iPhone Unlock. Next time you lock your phone, you'll need to enter your passcode to get back in. Unlike the above methods, this change sticks around until you go back into Settings and turn Touch/Face ID back on. (Your phone still remembers your face or fingerprint, so you won't have to teach it again.)
You can always turn off your phone. Whenever you turn it back on, it will require your passcode to unlock. So if the other methods aren't working for some reason, you could always reboot your phone to lock it.
More Extreme Measures
On Android or iOS, you can turn off fingerprint and facial recognition entirely, and just use a passcode. (On Android you can also use a swipe pattern, but make sure it's hard to guess.) Typing a passcode every time is a bit clumsy, but at least it keeps the keys to your phone locked up in your head.
If that's not enough, you can back up all your data, wipe your phone, and restore later. Of course, that makes your phone pretty useless in the interim.
Lastly, go through your settings to see what's available on your lock screen. If your calendar, text alerts, or other lock screen widgets contain sensitive information, disable them. No point locking up your phone if a new text can undo all that hard work.