Safe Car Park?

Statistically your luggage will be safer than practically all American cities, especially in a small village like San Gimignano. Most thefts occur in big cities.. Nevertheless it is always a risk to leave luggage inside a trunk. If you have no alternatives do this:
1. Items that you cannot afford to lose (e.g. passport, important documents, electronic devices, jewelry, medicines) take with you in a small backpack.
2. Items you can afford to lose but would prefer not to (e.g. your favorite pants, shoes, or shirt), hide in the spare tire hold.
3. Items you can easily replace (old tshirts) place in the cargo area all scattered around. Leave your suitcases open with nothing inside in the cargo area, all your belongings should be scattered in the trunk but not inside suitcases, because it’s very easy and fast to pull a suitcase out of a car and run away with it, but it is not as quick to steal all your belongings one by one.
4. Once you have done 1-3 park your car in one of the pay parking lots outside the old center (they are not guarded parking, there are only useless cameras). If you can, back your car against an obstacle (fence, bush, tree, pole) to make it harder to open the trunk (especially the spare tire hold). Be aware that most rental rental cars are hatchback or station wagons, therefore they can still access your trunk from the back seats (they just lower the back seat forward). But usually during a car burglary they would have time only to steal what can be easily grabbed once they lower the seat forward.

As I said theft in small villages is unlikely, especially because in SG there is a lot of foot traffic during the day with hundreds of tourists, including in the parking lots.

Advice I once received from a local: "If you see broken glass on the pavement, move on."