Looking at my achievements list in Galaxy I'm missing one "secret achievement". However checking against various achievement lists I do have all the hidden achievements in the game.

The only one I'm not seeing in Galaxy is called "There Was No Waldo". On the wiki and elsewhere it's listed as normal (not hidden) but since it's the only one missing in my Galaxy list I assume it's the hidden one?

Now what exactly is the requirement for this? The description says: "Explore all Frostland sites between the storms". I have done that multiple times but only in Serenity. It didn't pop. The description doesn't say it has to be done on Endurance but there are a few posts on various forums saying that's the case? Anyone can clear this up?