If there are no witnesses to a vehicle getting scratched in a parking lot but someone files a small claims case, what will happen?

Whomever is suing you has the burden of proof. That means that he or she must prove his or her case by a "preponderance of the evidence," or that it is more  likely than not that it happened the way that person says. The case he or she must prove is that you either negligently (carelessly) or intentionally damaged his or her car. That person can present his or her testimony about what happened; you can present your testimony, and your respectively credibility will be weighed by the court.

If your credibility is equal in the court's eyes and you both otherwise have the same amount (or lack thereof) of evidence, you should win, since you don't have to prove anything--but the plaintiff (the one suing you) does.