You might have noticed that when cops pull a car over they often touch its taillight or trunk upon approaching. They do that in order to leave fingerprints on the vehicle which would indicate the officer came in contact with the car. It is done as a safety precaution in case the officer is attacked or disappears as the fingerprints on the culprit’s car would serve as evidence. Today, most police cars have video cameras installed but, nevertheless, many officers would still practice the physical precautionary measure.
Another old police teaching practices include patting the taillight or the trunk in order to catch the driver off-guard in case they were trying to hide something illegal. However, nowadays police officers are advised against patting on the vehicle as it would give away their position thus making them an easy target for the potential criminals in the pulled-over car.