Ouch.
I have my dog in the car maybe three or four times a year. Because I rarely have people in the back seat, I keep one of these on the seat year round.
I can leave stuff on the rear floor or rear seat under the cover and from any of the windows, it can't be seen.
I've had my car broken into several times because my old neighborhood went south and I wasn't in the habit of locking up every night. They got my GPS, some tools and an expensive pair of sunglasses once. Other times it was just loose change and CD's.
I'm in a much better neighborhood now but still lock both cars and set the factory alarms (horn beeps once) before going to bed. I'm sure the neighbors are thinking "WTF? That isn't necessary here." but tahellwithem. I like to let anyone within earshot at that moment know that that they'll have to work for it.