So, random NPC just walking into my car picking up my stuff. Do I need to declare a camp or something to register my ownership? I don’t really want a follower yet, but I don’t want to have to blow his head off either.
They’ll wander in and take items that they want from any space in your vehicle that doesn’t have a cargo lock on it. If they wander into a base you have, they’ll take items, pretty much unimpeded.
They tend to do these things when you or your allied NPCs are not looking, so if you want to protect your assets, leave a guard behind.
Also, cargo locks can’t be put on every type of cargo space, so if you set up locks, make sure to put whatever is most important in a locked spot.
So … looks like I need to hunt down pocket watches for clockworks then.
I’m pretty sure the fancy watches and gold watches have them too.
In the experimentals, non-hostile NPCs aren’t supposed to steal your stuff, but there’s a weird bug that happens sometimes where they can.
I couldnt reproduce, so I guessed that items that had been hauled were the cause of some items not having ownership yet, and those were the ones the NPC were stealing, that potential cause is now fixed in experimental.
If OP is plying experimental, and NPCs are still stealing stuff that the player has moved previously and claimed ownership of, then idk what the cause may be this time 
If OP is playing 0.D stable or earlier then yes the first reply in this thread applies instead.
They violate the NAP, they get the AKM.
I’m on 9620 experimental. And, yeah, it’s a car I’ve fitted out with chemistry station / recharger / electric forge / minifridge / minifreezer and two cargo carriers. I’ve driven it around a bit as well.
BUT it could be he stole first then stepped into the car door, not stepped into the car door and then stole. Not 100% sure. He’s long since wandered off so no way to confirm.
In the future, two rounds of buckshot helps to stop NPCs from taking your stuff.
Bear trap the entrances to your vehicle.
This. Rob Keys used to plant caltrops around all the doors to the Sun Wave. It 100% dissuades NPCs from climbing in during the night, and you couldn’t get at the really valuable stuff from outside.
Caltrops have the advantage of taking much less cargo space than nailboards or bear traps.