My car was hit in a parking lot?

My car was hit while I was in a meeting at work. The parking lot has no surveillance. A man informed me that a green SUV backed into the front of my car; his/her tow hitch poked a hole in my left bumper. While I was surveying the damage, upset, the man left. Never having been in this situation before, I didn't think to get his statement or contact information. I should mention that my car was left unlocked as I was late to my meeting and rushed in. I made a claim to my insurance that evening and my car was repaired. 6 weeks later, I received a call from my insurance stating that another driver is claiming that I rear-ended him/her. Obviously, there is no police report. Since we both have two very different stories, what do I do? Can I be held liable, though they have no proof? They must have written down my license plate. My car was unlocked so they could have had access to any of my personal information. Please help!