https://www.bbc.com/news/world-us-canada-45011347

It is gratifying to know that while touching ones own face is suspicious, Air Marshals assure you that they won't arrest you for it. The US seems to be becoming more and more paranoid.

According to TSA internal documents published by the Boston Globe, air marshals observe passengers for a number of behaviours including:

Excessive fidgeting
Excessive sweating
Cold penetrating stare
Wide open, staring eyes
Face touching
How much they sleep during a flight
Using a smartphone

A spokesman for the agency said that if a passenger's behaviour is uneventful and the flight goes smoothly, they will not be approached or arrested.