Yeah but that would imply they are bringing the phones to the person multiple times to use their face/finger, or they are keeping the phone active so it never locks, unless they are actively changing the settings to never lock somehow. Seems like an easier fix to just require you to enter your pin to change your lock setting to indefinitely.
Side note: the last time I was arrested the officer asked me if I wanted to reboot my phone or turn it off before handing it over so I knew they weren't going to go through it. Was surprised