I just had the fun of finding one of my weapon trader settlers selling me my own alien blaster...

Checked my weapon container, and yep, no blaster.

Why we no have lockable containers?

Surely, it couldn't have been hard to program 'player only' containers...I don't recall any npc's helping themselves to my kit in previous games....

So far, I've had Carla knick a power armour...Curie help herself to all my fusion cores...some nondescript settler permanently borrow a minigun...and now this.

What's next, some npc getting my 'lover's embrace'?

Next level is, we'll walk in on settlers having their way with Piper, or whoever is your love interest.

Yes it happens, my own solution is I have my own home with no settlers/companions therefore no thieving (its got no doors either (-

If you prefer to live with settlers then perhaps build a bank vault style settlement somewhere else perhaps to store your precious goods?

Found one of my settlers with a power armor and another with a fat man. I've since moved everything into crafted containers and carry all the fusion cores just in case. Nothing has wandered since. Already provided containers and workbenches don't seem to be secure.

Crafted containers (i.e. one's that you build yourself that are not already in-place in the stock game) is the best solution. NPCs won't touch crafted containers. Every legendary weapon and armor I've found is stored in separate crafted containers in Sanctuary and haven't had a settler grab any of them.

Make sure to remove fusion cores from power armor. It's like leaving keys in your car.