ok, so i found a critical bug in Nokia 8, possible in other phones too...
it takes the google assistant only, and you can bypass fingerprint authentication or password input.
Just say Ok Google, and after it asks you to say something, press home botton, it will bypass the security and you can navigate freely in everyone's phone.
this happens in 8.1 april patch, and i hope you will fix it soon, it concerns my privacy.
Check if you have enabled the smart lock feature based on location / any Bluetooth device, Voice Match or others in settings.
I have a similar problem: I have both the google assistant and "ok google" detection turned off(both not even available in my language).In the settings the voice match is also turned off. Even the "smart lock" setting is turned off. But still the phones start a voice search at random moments multiple times a day when the Phone is locked. It happens random when the Phone is in font of me at work or when watching TV. Is there anyone that has the same problem or knows a fix?
Try and disable / uninstall the Google Assistant and see if this voice search happens.
Settings > Apps & Notifications > Advanced > Default application > Assist & Voice Input > Set as None.
Or If you are using Shazam application, disable the Auto shazam option.