My car was broken into at a trailhead a few years ago - I made the mistake of leaving a duffel bag full of my work clothes on the passenger seat. It was not worth anything to the thieves, but it was covering up a case with two laptops and $1000 of personal electronics. My SUV did not have a trunk to store items in. I thought it would be a safe place to park as there are always people there and I found a spot under a streetlight.
My insurance agent says he gets a lot of claims from customers who park at that location. It is near a crappy area of town and some apartment complexes. Typically one guy will walk between cars and smack the passenger window with a D-cell battery and keep walking. If nobody notices, his buddy also walks through and grabs whatever is in reach through the smashed window.
His advice was to pick a different trailhead in an area farther from the cheap apartments or preferably park on a nearby street in front of somebody's house. And of course, do not leave anything visible in the car even if it has no real value. I do not leave keys, wallet, or phone in the car. If you have a trunk, put items into it before driving to the trailhead.