I think we're pretty good at picking up things. In my opinion, the announcements are because there a people in the airport that bypass security everyday. Most of the airline employees such as mechanics, baggage handlers, gate agents etc, plus the airport employees that bypass security daily, maintenance, construction workers, etc. These people have access to doors that we don't and use them rather than go through security and there's nothing we can do about it. I've seen airline employees going in a door where you have to swipe your badge and they bring 4 or 5 people in with them that there is no record of. That is where someone will likely bring in something prohibited. They could bring in a bomb, put it in a trash can or whatever and leave. No one would know they were even there.