It is important to make a reasonable effort to notify the owner of another vehicle:

You must make a reasonable effort to find the owner of an unattended vehicle, or other piece of property, that has been damaged in a crash in which you were involved. If the owner cannot be located, leave your name, address, and license plate number in a note describing the crash.

• If you hit their vehicle while it is unattended.
• If you don’t like a bumper sticker displayed on their vehicle.
• That they are breaking the law while talking on their cell phone while driving.
• All of the above.