Since when playing the upcoming Survival rules will give Fusion Cores weight many players will choose to store some.
And, since NPCs can steal Power Armor if there is a Fusion Core in it.
And also, since NPCs can take items from storage containers in settlements.
Does this mean that NPCs will be able to take a Fusion Core from a storage container, put it into a player's Power Armor, and steal it?

I've still yet to have an NPC take anything from my storage or hop into a PA. And I leave my FC in them. Weird

pretty sure your workbench is safe storage. nothing has gone walkies from there ever for me.

I gave settler set to guard duty core in to inventory and left PA right next to him. He did not enter PA when under attack. Therefore I think your PA is safe. NPCs will use it only if it has core inside.

That is good news.
I don't use PA, but I do have a nice collection that I would like to keep safe. Hehe