Thermite/Thermate is metal, so therefore it would be detected by metal detectors.
It also has a unique signature that bomb sniffing dogs can detect, as would "puffer" machines.
My money is on terrorist "fat suits" which the full body scanner can be fooled. Body cavity concealment is an even more viable option. The last Batman movie gave a good idea; surgical implantation. The bad guys are always thinking of new ways of thwarting security efforts.
I guess full body x-rays, sniffer dogs and "puffer" machines are the best way to go then. X-rays are harmful if overused, and can damage DNA in everyone and are extremely harmful to pregnant women and children. Cancer anyone?
I think the main reason why the bad guys target the airliners is because if the airlines went out of business it could cause a multi-billion dollar collapse of the world economy. It would also cause such a disruption in world travel and it would affect business.
Oh man. It just gets worse and worse. I do like the idea of having ones on pilot license and private plane. *sigh*