I know it's difficult to come by, but suppose the bag he ended up using as his carry-on was also the bag he used in manuevers... and he worked with explosives in his military capacity. It could be fairly easy to forget about a block of it if he didn't clean out the bag thoroughly.
Whenever TSA says someone tried to sneak something through security, I tend to side with the passenger before TSA - like I said, I don't trust them.

What is the point of rules that are not enforced?