[FLGMwt]

Ah, yeah, that's what I was trying to get across

[DigitalJack]

When you record your face for the unlock, strike a pose that is not your default expression. Put your hand on your face with the sherlockian "hmmm" expression.

Guards/police won't know, they'll just hold the phone up to your face and try to unlock it.

[rhpistole]

Since it seems so much less invasive, I'd be worried the the arguments that you shouldn't need a warrant to do it will seem much more persuasive.