If there was a policy passed down to the thousands of homeland security officers at airports to screen phones for critical comments about Trump, is it plausible that not one person would leak that to the press? It sounds absurd.

We get ~200,000 foreign visitors flying into US airports a day. That we have a handful of people over months who had issues seems more like the normal rate, and evidence there is no weird screening policy, which would probably affect thousands, not dozens.