I never leave stuff in my car full-time in general; the GTO would be a horrible target for a thief, as the best they'd get would be an EZ Pass and an ice scraper.
The XC stuff is rarely contained, and gets schlepped out and back every time. I double duty the shoes for XC and commuting to work, and with the backpack and helmet soaked in sweat after a ride I'd hate to let that stuff fester in my trunk.
For DH, I have everything packed in a huge Dakine duffel, since I am only ride the mountain on weekends. It conveniently carries EVERYTHING, including armor and helmet, and after I get home I just wash it all, repack it and stick it in the downstairs closet.