Do you really need sources to assure you that advertising the fact, on your car, that you are a gun owner would be akin to an invitation for an opportunist burglar? I have been pulled over only rarely by the police for any kind of supposed traffic violation; however, some of the few times I was pulled over (when I was certain I had done nothing wrong) I had pro-gun stickers like NRA stickers on my car. One cop, after finding out I was a retired federal agent, commented on the sticker I had on my bumper in which the Gadsden flag was displayed along with the logo Unorganized Militia Propaganda Corps (Same one as at the link). He made every indication (without actually saying outright) that sticker was why I was pulled over. Another cop on another stop told me outright that sticker and an NRA sticker were why I was pulled over; he figured I had guns in the car (go figure where was the probably cause of a crime for the stop but that was in NYistan). Anything that brings unwanted attention can trigger a response from someone with an interest in what you display and crooks sure do like to steal guns. Whether its cops or criminals, I no longer advertise to avoid inadvertently attracting either.
Do I think crooks check out bumper sticker I doubt many do.
I think they walk up and see if the door is unlocked look at the pull or use the handle. If door opens and no alarm sounds they loot.
or
They walk up to car see free stuff sitting in the vehicle smash the glass grab and go.