I've been lucky so far, in 23 years of living near and hiking in the Columbia River Gorge. I've been told, though, that a lot of car clouters automatically go ahead and break windows, not bothering to find out whether or not the car is locked. Leaving the car unlocked therefore accomplishes only a considerable reluctance on the part of your insurance company to pay for anything taken (I asked my agent about this).
Do remember that not just loose stuff but built-in gadgets like a radio or CD player are worth something to folks looking to finance their next meth fix.
I can tell you that if you visit the Columbia Gorge, never, never leave anything (of value or otherwise) in the car. The same, I'm told, is true of trailheads along I-90 in Washington state. If you're traveling, arrange to leave your luggage at a motel if you want to go hiking. This is not quite so true of trailheads that are a couple hours' drive on bumpy dirt roads from major highways, but even they get hit occasionally.