just use the unregistered finger
use it a few times, and then it'll ask you to enter password.
I reach that stage even with a registered finger (touch ID is not super accurate).
optionally, you can try using using any finger other than thumb and index finger for daily TouchID use and when you are forced by the cops, use thumb/index finger for touch id. It'll look normal. 5 attempts with touch id is all you need.
use it a few times, and then it'll ask you to enter password.
I reach that stage even with a registered finger (touch ID is not super accurate).
optionally, you can try using using any finger other than thumb and index finger for daily TouchID use and when you are forced by the cops, use thumb/index finger for touch id. It'll look normal. 5 attempts with touch id is all you need.