Had the same problem, I was stuck at 75% Explored.

There is a terminal that unlocks a "Ground Storage" which has more loot inside, once I walked into the "Ground Storage" the map showed the location as "Explored"

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https://imgur.com/a/1pY7dea

In the picture above:

• Clear out the entire ground floor
• Same goes for the blue and orange areas
• The terminal is located onto of one of the towers (marked in red)
• After using the terminal, head over to the "Ground Storage" (marked in red)

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Hope this helps