This video shows you how to get the hidden banana in level 3 (3-3). The banana is in the leaves on the back side of the big banana tree. You’ll need to fling a yellow bird up at a very high arch to hit it.
Note, due to an error that occurred when recording this walkthrough we had to edit the last part a bit. You’ll momentarily see that it switches to another level, but there is indeed a banana here. We’re not intentionally trying to trick you, and no, you’re not in the Twilight Zone.