This was a weird glitch.
Happened to me a lot.
I used to have an empty power armor frame in sanctuary. Whenever i left it alone for a while, it started appearing as a full set of T45a, despite being empty. It started appearing as an empty frame again whenever i put a fusion core into it.
One time, in the south boston military checkpoint, there was a full set of X-01 power armor locked behind an advanced terminal. I got the second rank of the hacker perk just to get that power armor, and when i did so, it turned out to actually be T51 power armor that appeared as X-01 due to a glitch.
I have a building filled with empty sets of power armor in the red rocket truck stop. One time, they all appeared with seperate and seemingly random configurations of t45a parts.
I got a partial set of X-01, which i stripped to an empty frame and put in the castle. At one point, it appeared as a partial set of X-01 again, despite being an empty frame.
Anyway, I got myself a complete suit of X-01 in one go! Very nice, as I've just had the helmet so far, and haven't stumbled across any power armors in a while.
This was at the top of a building, just west of Custom House Tower. I had to fight off an Assaultron and a Sentry Bot at the same time.
That building (35 Court) was where i got my set of X-01 power armor too.
I think that's where a lot of people got it, since it's seemingly the only building in the game where (assuming you're a certain level) you're completely guaranteed to find a full set X-01, instead of random parts that might be X-01.
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