If they have reasonable suspicion that it indeed was. For example a case with a unique color pattern that is similar to a phone known to be stolen. Or if you ca

If they have reasonable suspicion that it indeed was. For example a case with a unique color pattern that is similar to a phone known to be stolen. Or if you can't unlock it. This ship has sailed quite some time ago - resist TSA or border control to unlock your phone, and they either confiscate it or turn you back. These powers are unlikely to ever be curtailed again in the name of security. But I also think that TSA and border control should rather focus on people actually violating immigration law and compromising travel security.