Probably admire it.
Personally, I wouldn't leave a rifle unsecure but if I saw one, I'd keep on walking. I don't find it shocking since it was common as a kid in PA to see rifles on a pickup window rack with the window down...sometimes in my school parking lot.
Times have changed!
No one would have peed themselves a little bit and ran for an adult? I just noticed it, did a double take, and drove off.
Bait?
So it wasn't a rifle in my story.... It was two small children. I was leaving the grocery store a few years back and noticed that a women drove up and parked in the fire lane... Getting out, she left a 6 and 5 year old in the car as she walked into the store. I waited 5 minutes and then walked up to the car and started a friendly conversation, nothing perverse, with the kids. Another 15 minutes went by and finally the mom came out. She gave me a rude look and the verbal beating ensued. I could not hold back my thoughts. Thoughts based in opinion and experience looking for missing kids. It takes less than two minutes to abduct a child. Most times I do not interject myself into others stupidity... That time I felt I had little choice.
Like everyone else.. looked and walked on..