There were another one afterall
Amazingly, I was completely unaware of this bonus level for over a decade. Thanks to Dr S for pointing this one out!
Before you get to the lobster at the end of this level, you have to navigate a network of stone tunnels. If you take the uppermost tunnel and keep going, it eventually leads to a dead end. However, if you fly into the wall at the end of the tunnel, it takes you to a bonus stage!
It's not too exciting if you've already figured out the molehill, though, because it's exactly the same as the Underground bonus stage on Level 1, albeit with more pondish music.
I wonder if that's all of 'em.
Gotta have everything in the video.