Unfortunately, it isn't only vehicles left overnight which can be hit. My pickup was a target on a winter day hike a few years ago on a hike up Willey. Had left it down on US302 as the road to the summer TH wasn't plowed. Some scum crawled into the back, opened the slider window, and stole just one credit card from my wallet - don't know why only one was stolen. They then went on a shopping spree at a Walmart in Connecticut. I'd had chosen Nieman-Marcus, but that's just me.
A friend of mine had a window broken on a winter day hike to Isolation 2 or 3 years ago. Nothing stolen, just the window smashed.
Of all the hundreds of times I've hiked and had friends who also parked their vehicles, these are only 2 instances. So, I don't worry all that much as it's a pretty low incidence. Still, I'm more careful about leaving items of value in my vehicles, and never leave my wallet anymore.
Personally, I find that I can't completely leave my vehicle empty. I usually have dry clothes, a shovel, tools, jumper cables, extra crampons, emergency gear, etc as it's just too risky to be without them. I do try to make sure they're not visible, however.