So many secretsG 5

Hard to miss. You get this after the Prologue if you chose to watch/play it. When you first get control and are walking to the left as go through the tutorial pop ups, I believe it is the second tutorial where it instructs you on how to fall below platforms, don't do that just quite yet. You should see a sparkle of gold just above that area. Just jump over to it and walk in to the Not -so-"Secret" area. Your achievement should unlock when you to claim it.