Please check your insurance policy carefully. This could be very tricky:
1. if you scratch the car by yourself and you have "Comprehensive Coverage", the insurance company may pay you the loss by deducting your "deductible";
2. like you mentioned the fact - if the car was scratched by unknown car, you may be not lucky because this case belongs to the "Unidentified Car Coverage", which may only cover the loss of properties except your car. This means you may get nothing from the insurance company.
That's why I suggest to study insurance policy each time carefully before you call them. If my understanding above is not accurate, please point out and help everyone here.