Due to the extreme difficulty with the latest trend in forum formats this is the 3rd attempt at this post.
Really? No cut n paste, no links? Getting silly guys, just plain silly to have these limits. I am sure there is some reason but yeesh taking the human element from a human interactive forum kind of defeats the purpose.
That said; I lost two sets of power armor either to glitches or NPC theft.
The first lost set cost me nine hours of gameplay and a full set of T51 fully modded armor. Yes, the save gap is my own fault as I got complacent with normal difficulty while cycling through fourteen settlements fixing the happiness glitch.
If it's a glitch that is. After the hoops I have jumped through on this forum; it wreaks of content stretching.
By the way, the prior posts were nice and to the point.
The second set confirmed my suspicions that the armor either vanishes or walked away by NPC theft.
Luckily the second time I had a recent save.
I created a small prefab wood building with four solid walls.
I can store four sets of armor inside and remove the one snap-in wall to retrieve them.
Even if a NPC somehow is able to enter the armor I don't think they can exit from inside.
For a visual depiction (no Spoilers) search YouTube for the CHANNEL "Gamer Gramps" and/or the video "Fallout 4 Power Armor Garage".