Particularly in clockwork levels, when you want to 100% clear the level, you'd often find yourself backtracking a long way. You'll often have to guess which way leads to the group button.
I found a clue in the mini-map!
See where the red arrow's pointing? That's a small object at the edge of a tiny room. That strange object is a lamp, illuminating a group button.
So whenever you see that small object, you know there's a group button!
And I thought you found a button to show the whole level's map :P.