If I were a terrorist, which I am not, I would strap harmless devices all over the place for a while, to get people used to looking at harmless devices strapped all over vital public infrastructure. Then, when it is time for the attack, the harmless devices can be replaced with harmful devices.
If, on one of my expeditions to place harmless devices all over the public infrastructure, I am stopped, I can howl bloddy murder about police over-reaction to my silly publicity stunt.
That's been working pretty well on airplanes, hasn't it? Harmless chanting, harmless conversations, harmless serial trips to the bathroom, harmless refusals to stay in one's seat. At the least, the thousands of unaffiliated but fervent jihadists who would like to "help" but lack means or will to blow anything up now have an answer to the question, "what can one brother do?" A tutorial for empowering nutcases and losers wanting to make an impact.