If your belongings were stolen because you left the car door unlocked, you'll probably need to replace everything on your own. While the fact that the car was unlocked isn't enough for your insurance company to deny your claim, it will make it harder to get your claim approved. Even if your claim could be approved, it usually makes more sense to replace the goods yourself.
Your car insurance won't cover the loss of your stolen belongings. It doesn't matter that your car was unlocked, these types of policies only cover your car and not the contents inside. While some car insurance policies cover items that are permanently attached to the car, like a built-in GPS or stereo, they do not cover regular personal belongings like luggage, iPods or laptops. If your car was at all damaged during the robbery, you can file a claim with your car insurance to repair the damage. However, if only your belongings were stolen, car insurance won't do anything.
Your personal belongings are covered under your homeowner's insurance. It doesn't matter where you personal property is at the time it is stolen, your homeowner's insurance will still cover it. If your belongings were stolen from your unlocked car, your homeowner's insurance coverage is the one that would pay for it. The terms and limitations of your coverage are listed under the personal property section of your policy. Read this before filing a claim as many personal belongings have limited or no coverage.
Before your insurance company will pay for your stolen belongings, it will investigate your claim for fraud. This is where you car being unlocked will be a problem. Your insurance company may suspect that you left the doors unlocked on purpose so that you could get money for your stolen goods. This makes it more difficult to get a claim approved than if your car was actually broken into. On top of this issue, you'll also need to prove that you owned the stolen goods. If you don't have copies of your receipts, you will have even more problems.
Most homeowner's insurance policies charge a deductible, which is often several hundred dollars. This is the amount you need to pay on your own before your coverage kicks in. The cost of the deductible will outweigh the value of most personal belongings. Even if there is something left over for your insurance to cover after you pay the deductible, your company might raise your premium as a result of the claim. Unless you left something extremely valuable in your car, it doesn't make sense to use your insurance to replace the lost belongings.