I'm always surprised by how many people think of their cars as mobile safes. They blithely leave backpacks and purses in plain sight in the passenger compartment, or, if they're a bit more security-minded, in the trunk.
Newsflash: the only thing keeping somebody from breaking into your car is laziness. Thieves will break into a car for just about anything. Somebody once broke into mine to take a paperback book.
Aren't they scared of crowds? Not really. How many streets are that busy? And even on busy streets, how many people will interfere, even if they notice anything's amiss?
Good thieves are efficient. Among a bunch of parked cars, they'll first target those that aren't empty. An empty interior is the first and best deterrent to a break-in.
What about the trunk? Well, if you can open the trunk from a lever next to the driver's seat or by folding down the rear seat backs, your valuables are no safer than if you left them on your seat. However, if you must leave stuff in the car, it's the best option you've got.
Put stuff in the trunk just before you leave your current parking spot. Thieves hang around parking lots and sidewalks to see who's putting goodies away. Leave right after you load your trunk and you literally remove their chance to steal from you. On the same principle, don't draw attention by opening your trunk when you park.
Are these precautions obvious? I thought so. But on the news I heard about a couple from Hawaii who left their backpacks in their rental car while they stared out at a beautiful view. The packs were stolen, of course. Hawaii being one giant tourist attraction, you'd think Hawaiians would know better. And they probably do — but vacations make fools of us all.