He left a letter for the car owner.
A closer look at the letter shows that it’s not only a few words of advice, a condom, too.
The note was very short and straight to the point. NICE PARKING. PLEASE DON’T REPRODUCE. REGARDS, HUMANITY.
Nothing else is known on what REPRODUCE means in the context. Could he be advising on birth control? Or was he talking about the driver parking wrongly again?