The image is common in films, and it also appears in real videos shared on social media. A police officer stops a car, and, as they approach the vehicle, they touch the trunk with their right hand. The habit is common enough that it is clearly not a coincidence.
The gesture is part of a protocol used in the United States, among other countries. The police officer approaches the trunk of the car on the driver’s side and places one hand on the back end. By doing so, they ensure that the trunk is closed and that no one will jump out of it.
But the action goes one step further: it is meant to ensure the officer’s own safety. Touching the trunk in a defined area leaves their fingerprints on the car. If they were to be attacked, the vehicle could be linked with the crime.
The officer approaches the driver from the side to avoid being run over if the car drives forward or backward. Once at the level of the driver’s seat, the police officer also tends to put their hand on the roof, just above the window. That ensures that their fingerprints will be visible in yet another area if needed.
So the gesture, so ubiquitous in videos and films, works for security purposes, allowing the officers to leave evidence of their interaction with the vehicle in case of a serious incident.