Acorn Plains has a secret exit, each world does; Soda Forest has multiple. There's no indication on the overworld which ones have an exit, aside from a suspicious blank spot on the map.
If you'd like to know the number of the stage in Acorn with the secret exit, it's1-2.
Thanks. I think I OT annoyed with NSMB2 (or 3d land, can't remember) which was the same. I'd much prefer some indication of secret exits. They and the star coins are often well hidden without making you scour every level.
Still, now I know, I can be on the lookout.