Ok. What's done is done. You've no doubt learned some good lessons about handgun storeage. I won't lecture.
Provided you've reported the serial number on the gun as stolen, you shouldn't incurr any criminal liability. But, that doesn't mean that you can't get "drug into court" to explain to the jury exactly WHERE the crackhead GOT the pistol. ("He stole it out of my car, sir!" "The witness is excused...")
Civil liability is another matter... If some kid stole it, and he uses it to kill either himself or someone else, you could get sued for negligence for failure to exercise care in storing your gun, and leaving it where some kid could get 'hold of it. Now, I'm pretty sure that you'll have a valid defense due to the fact that the gun was actually stolen, and not simply left laying about. But, by the time you'd get up to presenting your defense, you'd have already spent several thousand dollars in legal fees.
I don't believe you've committed any crime. It was foolish to leave a gun in a car, and I think you know that now. But, just because you didn't commit any crime DOESN'T mean that you can't get drug into a courtroom to explain HOW you didn't commit any crime.
For now, there's no reason not to think that your pistol is sitting in some pawn shop somewhere right now. If it is, then that's good news for you. Even if the cops don't identify it as stolen and return it to you, by allowing the pawn shop to sell it, title will pass to the purchaser, and you will be removed from the chain of title.
If the gun is used for a crime, then first they have to FIND the gun. Then they have to TRACE the serial number. THEN they have to come find you. THEN they have to decide that it's somehow YOUR fault that the gun was stolen. THEN they have to file charges against you. There are alot of uncertainties in that scenario. Right now, I'd just chill out. Call your attorney and tell him about it, but don't hire him outright just yet. Call your insurance adjuster and tell him about it. There's no need to build up a rampart of hired-guns at this point. Take it easy, and wait and see what happens.
Oh, and from now on, keep your blankety-blanking handguns locked up in the house.