They still find dinosaur bones too. I guess that proves it's really hard to find 100% of anything. But it doesn't prove that it is hard to find 1% of something, so the analogy to weapons in Iraq is not very appropriate.
Actually, even though I'm very anti-war, I expect them to find some chemical weapons in Iraq eventually. Even if Iraq was trying to get rid of them, it is unlikely that they could have gotten rid of every little shred. After all, they still find unexploded shells in northeast France.