The area we were able to effectively explore in earth seemed to be fairly large, but you can traverse it pretty quickly. Even so my brother wandered into an area with level 20 dregs on earth so I'm sure there are area's meant to later story mission or strikes at high level.
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I noticed several times as well in some of the shoreline caves and an area inside the barrier on Earth. I wondered if that was accidental that forgot to remove for the Beta or didn't expect anyone to find it.