It'll be possible to make secret levels unlocked by getting an achievement, rather than by finding a secret level trigger. You might want to consider using this option instead. If you do want to use secret level triggers, it's probably a good idea to make some subtle hint in the terrain as to where they are (perhaps something that looks out of place, or a platform that serves no purpose, or a place that very obviously is challenging to get to yet not involved in the main solution).