Wow... Someone absolutely nailed it in the comments...
"If you DON'T make a gesture...are you carrying a concealed weapon?"
No, that's only if you make the gesture under your jacket, so nobody can see it.
Durkin, however, said the TSA eventually let her off with just a warning, after she assured them she learned her lesson.
A warning? To do (or not do) what? Make the school's well-known hand gesture?
TSA -- Terribly Stupid aholes.