I saw this thread, so i did not leave anything in my car that was valuable when i went to the quarries today. Of course though, my car window got smashed in. Not sure if the alarm scared them off, or if they were just smashing windows for fun, but it didn't seem like they stole anything. Some neighbors said it is a reoccurring problem, and often nothing is stolen.
If i was to go again, i would probably want to park on one of the side streets. The nice woman even said she would watch my car for me next time, and to park it right in front of her house.