While not truely surreptitious, if you can’t reach your phone for whatever reason but you do have Siri enabled you can also disable Touch/Face ID with 2 voice c

While not truely surreptitious, if you can’t reach your phone for whatever reason but you do have Siri enabled you can also disable Touch/Face ID with 2 voice commands. First “Hey Siri lock my phone” This command screen locks the phone but doesn’t disable Touch/Face ID - you can skip this phrase if the phone is already screen locked, however it does no harm if the screen is already locked, so might be handy to just say it anyway esp if you can’t see the screen to verify it’s screen locked. Then “Hey Siri whose phone is this?” This command will say the first name of who the phone belongs to and if the phone is screen locked disables Touch/Face ID. You can also switch off the phone with 2 Siri commands which will also disable Touch/Face ID. “Hey Siri, turn off my phone” or “Hey Siri, restart my phone” Siri will ask you to confirm the power down/reboot, so confirm with “Yes”. The phone will then power down/reboot.