It’s circa 1990, and there is a big snowfall in Some City, USA. The mayor has left his car parked in a public parking lot, and the next morning, he discovers that the lot has been plowed, but the snowplow driver has buried the mayor’s car under a large pile of snow. The mayor calls the police to figure out whodunnit.
A police officer comes to the scene, who radios police headquarters, asking them to call the public works department. Public works shows no record of any city employee having plowed the lot, so the officer suspects a private citizen may have plowed the lot and buried the mayor’s car as a prank.
The officer finds no eyewitnesses to the plowing, and there are no security cameras. The officer looks around the parking lot for clues. There are some ordinary-looking snow-tire prints left by the snowplow truck on the snowy ground, there are some ordinary-looking plow marks along the ground and in the snow piles, and there is a patch of black exhaust soot in a snow pile where the truck backed up too far. There is no indication that the perpetrator left the vehicle while at the scene. The officer considers calling forensics to try to match the tire or plow marks to a truck or snowplow, but that would be costly and time-consuming. Other than the exhaust soot, the offending party has not left any parts at the scene, such as fenders, paint flecks, cigarette butts or dropped business cards.
Then the officer sees something else at the scene, makes a quick radio inquiry to police headquarters, and says, “Mayor, we know who owns the truck that did this.” What did the officer see?
This may sound like a "What am I thinking?" puzzle, but there is an "Aha!" answer in the story.