Two points. One: out of site means they won't know which car to take things from.
Two: patrol cars are unattended fairly often. There are rare thefts out of cars, but the ratio is probably close to thefts from officers homes.
Most cars are usually under more scrutiny in public so they may actually be more secure, depending on the situation.
You can only make anyplace just so secure, depends on the motivation of the thief.
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