So my phone (Marshmallow) was locked. I said Ok Google, to search something. It searched directly from the lockscreen and when I tapped home, it actually unlocked the phone.
Is it a security hole or something?
My phone is Moto G4 Plus
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Trusted voice ("OK Google")
When you say "Ok Google" from a secure lock screen and we're able to recognize your voice, you can ask Google to do things for you, or visit sites, without manually unlocking your device.
"OK Google" settings options (How to turn on/off OK Google and its settings (Trusted voice, etc)): Google Support
It is a feature as you can see in Lenovo forums
Long press on home screen and then press on settings.
Go to voice -> ok google detection and then turn off trusted voice.
Another post there says
Its not a bug. It just recognizes your voice but its not safe. As the feature is not accurate it may unlock even with others voice. Better turn off trusted voice. The voice command works even from lock screen works if your turn on option to Ok google from any app.
It's part of smart lock as trusted voice. I'm using trusted location as my smart lock. Whenever I'm at home and office phone get unlocked automatically when pick it up. My home and office co ordinates are as trusted locations.
You can also use trusted devices, such as your fitbit, Bluetooth devices. Whenever you make phone contact with these devices phone get unlocked.
I liked this feature a lot, more convenient to unlock phone. But with caution. Keep backup security as pin or pattern.