They weren't hidden
As bizarre as it sounds, at the time, it was permissable to carry box cutters on a plane, either on your person or in your carry on bags. The small blades weren't seen as a threat. However, in the hands of someone who knows that they're doing, they can be deadly weapons. The hijackers simply brought them on with them.
As for your crack about propagandists, several people on the planes managed to make phone calls before the planes crashed. They told what was going on to people on the ground. Don't tell me you're one of those conspiracy nuts.