That’s what the mounted M2 Browning in an automated turret chassis is for. Or just a locking door.
I mean, I don’t object to a car alarm, I do object to the requirement of having one to mark my stuff as “off limits on pain of death,” especially since very often the vehicle you want to secure is in fact a shopping cart which you transport on your truck and reassemble when and where you need it. That’s just something else to weld on and remove when you’re moving on.
A cardboard spray-paint sign that signals death to thieves and marks any npc willing to still steal as hostile on touch of items should be enough. Identical flag should apply to any vehicle currently BEING HELD by player. I don’t care how oblivious to ownership or anarchist you are, I will discuss you not stealing the stuff I looted and give you firm verbal warning to steal at the risk of a shotgun to the face.
On that topic, a player version of the “drop your ___ and surrender” dialog, and the “this is my terf go away” would be nice to have for when they grab that item you left outside your zone, or where just about to grab yourself, or forgot to mark your zone, that particular npc just rubs you the wrong way, or you just want to be the bandit for once, would be really nice.
Player signs could be used to inform neutral/hostile NPC of a player zone (could use the actual zones to make them more useful.)
Honestly, I’d rather it be something you could just abstract with zones, rather than forcing the player to go to some artificial step to make it official.
“Hey! That’s my pile of stuff, in my fortified house, in my zombie-free zone, behind my moat! What the hell do you think you’re doing taking my katana and pillow?!”
“I don’t see your name on it! You didn’t erect a nerdpole, so it’s not officially yours, so you’re gonna be the bad guy if you kill me to get it back!”