When I worked at Destinations Cyclery, it was located at the corner of Parker Road and Cottonwood
Drive. The King Soopers grocery store (Kroger) was across Cottonwood from us, so we saw a lot of traffic coming out of their parking lot and turning onto the highway.
Over the course of one summer, when I was bike-commuting pretty regularly, I found out that a lot of people place their purse, daytimer, or wallet on the roof of the car, while unlocking the door and loading groceries into it. How did I find this out?
About 3 nights out of 5 I would find either a purse, daytimer or wallet on the road as I bicycled home. Almost every time that I contacted the owner, I would get the same story:
"I must have put it on top of the car at the grocery store..."
I would say that putting your valuables on top of your car is not really a big deal. Driving off without putting them inside the car, however...