This video shows you how to get the hidden banana in level 30 (4-15). The banana lies in the bush in the bottom center. Watch as a rock rolls into the bush activating the banana. This one was found by accident in the video, so it may not be the best way to get it. Annotations were added to the video to help show the location.
If you are only showing 14/15 bananas even though you’re pretty sure you got them all you may want to try and get the banana on 4-14 again. The crate is no longer labeled, but a few people have reported that after breaking it they show 15 of 15 banana. Thanks to Ivan for being the first one to point this out.