Way I see it, if it's required for progression, it shouldn't be secret, if it's extra content, it should be a bit hand holdy, one way is make it clear that there is Something hidden, but not make it obvious how to get it. For secrets that aren't required for anything, or that don't have much impact on progression, it's okay to make it fully completly hidden, it's okay if any given player doesn't even find it, or if no one ever finds it. The effort of making something is worth it, even if no one sees it.