A lot of people cover their VIN. If you have the number you can get keys made for the car. At least that's what I always heard. As far as the serial number on a gun goes it's nobody's business. I don't like to advertise mine.
Not the newer cars. But I get your point. I've just never seen the danger in anyone seeing it. (VIN or firearm serial#). If you stole either the gun or the car, that's obviously a different story. And as far as the newer cars, it's all but impossible to get keys made, because most of the vehicles that still use them have chips imbedded into them. The days of going to the hardware store or a kiosk, and getting extra car keys made are over. Only the dealers possess the blanks... And they're very costly. Others, like our new 2015 Jeep Grand Cherokee, are completely keyless. They require you to have the remote key Fob in your possession, in order to enter the car and start it via pushbutton.
Even if you exit the vehicle with the engine running, and return without the Fob, it won't allow you to put the car in gear. We got 2 Fob's with the car. And they told us if we lose both of them the vehicle has to be towed to the dealer, have 2 new Fob's issued to it. And then have both the computer reprogrammed by the dealer, along with both Fob's...... Cost? Around $2,000.00. And they won't do a thing until you prove to them it's your vehicle. And it's a very costly tow because the entire drivetrain, doors, hood, and hatchback are all locked up tighter than Ft. Knox. They have to jack it up and put dollies under it just to move the damn thing, and get it up on the carrier. Not to mention the alarm system is armed, and the battery is under the passenger seat. And the Fob's themselves are very durable. Mine already went through the washing machine once. It didn't effect a thing.
I've seen extra Fob's on sale from Amazon for around $150.00 each. The problem is they're useless unless the dealer programs them. And if they didn't sell you the Fob, they're going to rape you on programming it. Much the same as if you brought your own parts and wanted the service department to install them. The manufacturers have theft pretty well covered. If you look at the top stolen car list, a lot of them are older models. (Mid 90's to early 2000). Simply because they're much easier to steal.