I assume nothing.
People often buy used cars with stickers already on them.
Often those same people are never sufficiently motivated to remove them. (It’s work)
I don’t make any assumptions.
I am embarrased and really hate to admit this, but as someone who worked in HR and in-house recruiting, such a sticker would make me less likely to hire you. I would be tempted to make other generalizations about the individual (anti-gay, perhaps a bit uncomfortable around minorities, not likely to treat female co-workers as equals, etc.). I know its wrong and I shouldn’t, but its just hard not too.
Not that there are many vehicles with NRA stickers in San Francisco. And a confederate battle flag? Hell, I’d call the cops and evacuate the building. 
If I saw a Volvo with an NRA bumpersticker I’d want to meet the driver.
First of all, I’ll say I don’t like guns at all and I really dislike Mr Heston.
S1, Job Interview
-no difference
S2, highway
-There isn’t any way you’re pulling in front of me on the highway, I drive far too fast for that.
S3, giving me a ride to lunch
-Honestly, I’d probably be a little afraid to begin with and ask you a lot of (innocent) questions about it. See, I never even saw a gun in real life until I was like 17 (except maybe holstered on a policeman).
What if it was a black female lesbian who had the NRA decal on her car?