You're absolutely correct that there's nothing stopping your proposed scenario from happening. That being said, it's extremely unlikely they'll do that because

You're absolutely correct that there's nothing stopping your proposed scenario from happening. That being said, it's extremely unlikely they'll do that because in order to do so, they would have to: 1) Intercept your emails 2) Store the fact that you're a 1P user 3) Match up your email address to you, as a person at the border 4) Stop you at the border for questioning 5) Force you to unlock your phone 6) Recognize that your phone does not have the 1P app installed 7) Force you to install the app + unlock it For certain people, they may do that. But for the vast majority of people, they will not. 1P does not enjoy the popularity that Facebook is at. In the western world, one can reasonably expect any random given person to have a Facebook profile. The same cannot be said for 1P.