They are also ninjas. I have several plants that I KNOW must be hosting hornworms -- I see the disappearing leaves, a small grazed bit here and there on green fruits... But CANNOT for the life of me find them.
...of course, sometimes, it's because the birds have been there ahead of me. Usually, if I hear the cardinal hunting, I can relax.
Learning never ends because we can share what we've learned. And in sharing our collective experiences, we gain deeper understanding of what we learned.