We spent ten days in a Renault Safrán in 1999 and drove all over with the luggage under the "tarp" in the back with no problems. We did unpack the car EVERY night when we went in, and never left things out in plain sight. I can't imagine how leaving the car unlocked is conducive to avoiding crime. They may just take it for a joyride.
Usually the casual criminal is looking for an easy mark. If they have to choose between the prize they have (i.e. what they can see) and what's behind door #3 (i.e. what "might" be in your car), they're going with the bird in hand.
We also parked in public underground parkings, which helped things a bit, as access is a little more controlled, and you are "out of sight, out of mind" to many of the crooks.
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Ongi etorri!