Inside a business building is one thing, but in the car means that there are two property rights to balance. Does the owner of a parking lot have a right to know what is inside each car? I don't think so. Sure, they can deny access to anyone they choose, but they don't have an automatic right to know or even ask what is in the car.
I can, for example, allow or not allow someone to park in my driveway but that doesn't mean I can search their car, or even have a right to ask them what's in it. If I can't ask them what's in it then I've also got no business demanding that it contains no guns.
There are competing property rights here. I would submit that contents of cars are out of bounds.
You mean if I let people park on my property, it's no longer my property? Or at least, my property rights are "balanced" by someone else's equal-and-opposite right to use my property against my will? Interesting.
I assume the same is true of your driveway? So I can walk up your driveway with an M16, as long as I don't leave the driveway or enter your house? Because my rights and yours "balance" in your driveway? Just curious.
There are competing property rights here.
Property rights never compete. Your property is yours, and mine is mine, period.
You can do both of these things- you can tell visitors to your property that their using your driveway is conditioned on allowing you to search their car. They can refuse a search, but you can deny them the privilege of parking on your driveway.
Many private parking garages here in DC, for example, require you to pop your trunk and submit to a search of your car before letting you in to park. Perfectly legal.