Taking photos of your phone screen is the main loophole and is completely undetectable. Exactly what happened to Waltz and what caused TFA.
If you really need to, you can combine this with a rig that holds the phone and the camera just right, controls the lighting, and interacts with the phone via a hotdog mounted on a gantry. Come to think of it, any 3D printer can be adapted to archive Signal/Snapchat/etc. messages in a completely undetectable way. Could even reply if you rig up another phone to talk to your hot dog finger + camera robot.
