I don't know. I don't trust TSA any farther than I can throw them one-handed. They are evidence that government contracts go to the lowest bidder, IMO.
While I find it weird to accidentally have a block of C4 in one's bag, I suppose it's possible. That's why when I travel, I always clean out the bags before packing - every compartment, every pocket. And wouldn't it be amusing to find out that the block of C4 travelled to Midland in the carry-on, as well (as in, TSA missed it the first time)?
I know several people who have gone through security only to find a stray (unfired) round of ammunition in their bag. Of course, this causes a complete circus at the security gate.