So with MB's recent "exceedingly rare" comment, my feeling is that if they're so rare, it's unlikely to require the mission as a prerequisite for them to spawn. Of course I may be totally wrong, it's just one of those gut feeling things.
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I agree with all of this more or less. Although the POI spawn rate is likely lower than 0.25% or statistically I should have found it by now a few times over. I think it could just be incredibly low, but based on the assumption that with enough people looking at POI's it will eventually spawn for someone.
The only problem I have with this is that they made a mission to find it, I'd almost have preferred knowing nothing about the mission and finding that someone has stumbled across it one day.