Yes, I know they go into inventory when they need repair. That isn't the issue.
I will park my PA on top of RR and come back only to find a frigging leg or arm piece missing? Wth?
Not in inventory, had just repaired all bits, took core and left.
Came back, voosh, gone.
Do settlers steal the parts?
This has happened two or three times.

never had anything steal a PA piece as long as core was out.

may want to move your ladder back so a small jump is required to get to your PA to be safe..

Figured it out. Feel pretty stupid.
Parts can be damaged and then knocked off.
I watched it happen to mah leg. 馃槚

Leg was red in HUD, was making my way back to base and got ambushed by RadScorpions.

I thought the damaged parts went into inventory. They don't. I saw them fly off and could see them on the ground but could not pick up.

that's a bug and not intended behavior.

Never had PA parts fall off, they always end up in my inventory after they get damaged.

What I've seen is that unused Power Armor frames might start showing pieces that they don't have equipped, which eventually disappear.

I always seen parts blow off when durability hit zero but it still goes into inventory as the flying parts are just visuals for the broken armor and not an actual item you can pick up. I shoot the limbs of dead power armored enemies all the time to drop the durability so they become weightless and they were never able to be picked up by the flying scrap.

The randomly vanishing limbs is something I never seen though.

Hmmm. Is it truly a bug? That would make sense because, yes, usually the parts go into inv and I can repair and equip at workstation.
Dang it. Lol, have five sets that I found w/o cheating; all missing a leg or arm.

I am almost positive that in the one instance (original post) the armor was all good to go with nothing missing when I left, not sure what happened there. I could be mistaken but I did see it blow off and vanish with the other. 馃槩