I don't know your neighborhood, but I've been leaving a trunk rack on my car most of the time for years and it's never been stolen. I think about it sometimes, but so far, so good. This includes leaving it on the car in the city and suburbs, including some 'out of the way' spots where a thief would definitely be undisturbed.

I think theft is possible, of course, but unlikely as racks aren't worth a ton of cash to most folks. (looking on CL, most are priced well below new). Having said that, I fully expect my rack to be stolen next time I leave it on the car.