Many petty thefts are crimes of opportunity. If something is left in plain sight and it's easy to nab/steal, there's a higher chance someone's going to grab it versus whether or not it's left under lock and key.
Depending on the "area" and how bold certain thieves are in certain areas, you might have to take extra precautions. Years ago I left two backpacks in the backseat of my car while I hopped on a train to NYC for a quick trip. I was gone for a total of three hours from when I boarded the train and lo and behold, when I got back to my car, my back window was smashed to bits, both of the backpacks - gone.
I was more upset that I now had a window to fix, but I should've known better - I did leave my car in Newark. Whenever I kept stuff in my trunk, I never had a problem because out of sight, out of mind.