This is one of those details we need for good interaction with random NPCs, yea.
For the, “sleeping in the car” case, the question is what’s a good ui, do we make it manual, or just assume you lock doors any time you close them from the inside? I’m leaning toward making it automatic, mostly since the number of moves to do it would be tiny, and theres little to no reason to leave them unlocked.
It gets a little involved to handle locking car doors from the outside, generally we assume you break into cars and hotwire them, so you don’t have a key. To handle this we’d need a system for tracking keys (Not hard) and a system for placing keys on the map (tricky, I don’t have a good solution). If your character is good at lockpicking, you might want to lock car doors after closingbthem from the outside as well, but we can’t assume that’s safe to do, so it’s need to be manual or configurable.