I think this is broadly not true. Sure, the NSA can probably pull this off. Thing is, the NSA probably does not need to do this at immigration. I seriously do

I think this is broadly not true. Sure, the NSA can probably pull this off. Thing is, the NSA probably does not need to do this at immigration. I seriously doubt that this is a realistic problem if your threat model is anything less than "The NSA is very interested in me". In that case I don't see how you could trust any phone, regardless of it having been in the hands of border officials or not.