Someone possibly keyed my truck - What do I do next?

So for context, Monday night I received a note on my windshield when I left my girlfriend’s apartment. It read “At least park on the other side you don’t live here” What I immediately took was they expect me to park in visitor parking. Which I normally don’t because I have to walk quite a ways, so I normally park near my girlfriend’s apartment building. Upwards of two years this has not been a problem, no complaints, and have never been towed by her apartment complex.

Her apartment has covered parking, which is extra to pay for, and VIP parking (closest to the building) which is also extra. The spot I was in was literally the closest one to my girlfriend’s apartment, right down the alley way. The spot was not VIP, or covered. So it was a normal spot for residents. Which I admit normally I never park that close because it’s taken, but it was open when I arrived that day.

Tuesday evening comes and my parents both walk into my house and say “Hey, you have a huge scratch on your truck” I was bamboozled because I didn’t remember hitting anything or something other than rocks slinging up. Sure enough I see there is a scratch the whole side of my rear drivers side door. I drive an F-150 crew cab, so the door is about 3 feet wide. So you can picture it. It’s not like a rock just got slung up and glided along my truck.

I contacted the apartment complex, and the local PD and basically said “I’m not sure it happened here but using context clues that I had a note that someone was clearly irritated enough to right and a scratch that was noticed the next day.

Is there any advice on how I could deal with this better?

Thank you for any responses before hand.